Olfactory deficit is a potential functional marker across the Alzheimer's disease (AD) continuum. Alzheimer's disease is a prevalent form of dementia affecting 32 million people globally. Early identification of biomarkers is crucial for screening at-risk populations and timely intervention. Olfactory dysfunction, which indicates a decline in the ability to detect, identify, or remember odors, precedes detectable cognitive impairments, including mild cognitive impairment (MCI) and subjective cognitive decline (SCD), by several years. Olfactory impairment is closely linked to clinical symptoms and neuropathological biomarkers of AD, with significant structural and functional abnormalities in the brain. Olfactory behavior examination and olfactory functional magnetic resonance imaging (fMRI) can provide objective assessments of olfactory capabilities, potentially becoming a promising tool for exploring the neural mechanisms of olfactory damage in AD. This review summarizes recent literature on the characteristics, neuropathology, and examination of olfactory dysfunction in the AD continuum, focusing on early changes in olfactory indicators detected by behavioral and fMRI assessments and discussing their potential in MCI and preclinical AD. Despite challenges and limitations, olfactory dysfunction has demonstrated its value in assessing neurodegenerative diseases and may serve as an early indicator of AD in the future. Olfactory dysfunction is prevalent in the AD continuum, with factors such as aging, neurodegenerative diseases, head trauma, and exposure to toxins contributing to olfactory deficits. The olfactory system is affected early in AD, impacting both olfactory and cognitive functions. Olfactory dysfunction is associated with increased risk of MCI and AD progression. In the preclinical AD population, olfactory dysfunction is linked to increased risk of future cognitive decline and progression to MCI or dementia. Olfactory neuropathology in the AD continuum is closely related to core AD biomarkers, with Aβ and tau pathology accumulating in olfactory regions. Olfactory fMRI reveals functional deficits in the AD continuum, with abnormal activation in the POC, hippocampus, and insula. These deficits are closely related to altered cognitive function. Olfactory fMRI offers objective assessments of olfactory function, with advantages including synchronous collection of image-odor stimulus signals, high data resolution, and co-analysis with functional-structural data. These advantages assist researchers in focusing on changes in olfactory-specific brain regions in the AD spectrum. Olfactory fMRI has the potential to become a promising tool for exploring the neural mechanisms of olfactory damage in AD.Olfactory deficit is a potential functional marker across the Alzheimer's disease (AD) continuum. Alzheimer's disease is a prevalent form of dementia affecting 32 million people globally. Early identification of biomarkers is crucial for screening at-risk populations and timely intervention. Olfactory dysfunction, which indicates a decline in the ability to detect, identify, or remember odors, precedes detectable cognitive impairments, including mild cognitive impairment (MCI) and subjective cognitive decline (SCD), by several years. Olfactory impairment is closely linked to clinical symptoms and neuropathological biomarkers of AD, with significant structural and functional abnormalities in the brain. Olfactory behavior examination and olfactory functional magnetic resonance imaging (fMRI) can provide objective assessments of olfactory capabilities, potentially becoming a promising tool for exploring the neural mechanisms of olfactory damage in AD. This review summarizes recent literature on the characteristics, neuropathology, and examination of olfactory dysfunction in the AD continuum, focusing on early changes in olfactory indicators detected by behavioral and fMRI assessments and discussing their potential in MCI and preclinical AD. Despite challenges and limitations, olfactory dysfunction has demonstrated its value in assessing neurodegenerative diseases and may serve as an early indicator of AD in the future. Olfactory dysfunction is prevalent in the AD continuum, with factors such as aging, neurodegenerative diseases, head trauma, and exposure to toxins contributing to olfactory deficits. The olfactory system is affected early in AD, impacting both olfactory and cognitive functions. Olfactory dysfunction is associated with increased risk of MCI and AD progression. In the preclinical AD population, olfactory dysfunction is linked to increased risk of future cognitive decline and progression to MCI or dementia. Olfactory neuropathology in the AD continuum is closely related to core AD biomarkers, with Aβ and tau pathology accumulating in olfactory regions. Olfactory fMRI reveals functional deficits in the AD continuum, with abnormal activation in the POC, hippocampus, and insula. These deficits are closely related to altered cognitive function. Olfactory fMRI offers objective assessments of olfactory function, with advantages including synchronous collection of image-odor stimulus signals, high data resolution, and co-analysis with functional-structural data. These advantages assist researchers in focusing on changes in olfactory-specific brain regions in the AD spectrum. Olfactory fMRI has the potential to become a promising tool for exploring the neural mechanisms of olfactory damage in AD.