This study investigates the association between obesity and cognitive decline in middle and old-aged Chinese adults, focusing on the mediating role of physical activity. Using data from the China Health and Retirement Longitudinal Study (CHARLS), the study analyzed 7,392 participants aged 45 years and older. Cognitive function was assessed through episodic memory and mental status, and Group-based trajectory modeling (GBTM) was employed to identify cognitive function trajectories. Multivariable ordinal logistic regression was used to examine the relationship between obesity and cognitive trajectories, with body mass index (BMI) as the indicator of obesity. Mediation analysis was conducted to assess the potential causal chain where physical activity mediates the relationship between BMI and cognitive decline. The results showed that obesity was associated with a lower risk of cognitive decline compared to normal weight, with an adjusted odds ratio (OR) of 0.81 (95% CI, 0.70–0.94). However, only vigorous physical activity significantly mediated 5.94% (95% CI, 0.29–11.60%) of the relationship between obesity and cognitive decline. Sensitivity analyses in different subgroups confirmed these findings. The study concludes that vigorous physical activity mediates less than 10% of the association between obesity and cognitive decline in middle and old-aged adults. Further research is needed to explore other factors that may influence this relationship, particularly in diverse cultural and demographic contexts.This study investigates the association between obesity and cognitive decline in middle and old-aged Chinese adults, focusing on the mediating role of physical activity. Using data from the China Health and Retirement Longitudinal Study (CHARLS), the study analyzed 7,392 participants aged 45 years and older. Cognitive function was assessed through episodic memory and mental status, and Group-based trajectory modeling (GBTM) was employed to identify cognitive function trajectories. Multivariable ordinal logistic regression was used to examine the relationship between obesity and cognitive trajectories, with body mass index (BMI) as the indicator of obesity. Mediation analysis was conducted to assess the potential causal chain where physical activity mediates the relationship between BMI and cognitive decline. The results showed that obesity was associated with a lower risk of cognitive decline compared to normal weight, with an adjusted odds ratio (OR) of 0.81 (95% CI, 0.70–0.94). However, only vigorous physical activity significantly mediated 5.94% (95% CI, 0.29–11.60%) of the relationship between obesity and cognitive decline. Sensitivity analyses in different subgroups confirmed these findings. The study concludes that vigorous physical activity mediates less than 10% of the association between obesity and cognitive decline in middle and old-aged adults. Further research is needed to explore other factors that may influence this relationship, particularly in diverse cultural and demographic contexts.