This study investigates the effects of light intensity on the growth and flavonoid synthesis of celery (Apium graveolens L.) under hydroponic and full LED light conditions. Four light intensities (40, 100, 200, and 300 μmol·m⁻²·s⁻¹) were tested, with three harvest periods (15, 30, and 45 days after treatment). The results showed that increasing light intensity generally promoted celery growth, with the highest increases in plant height and aboveground biomass observed at 200 μmol·m⁻²·s⁻¹. Light intensity also significantly influenced the content of chlorophyll, carotenoids, total phenols, vitamin C, cellulose, total flavones, and apigenin in celery. The activities of key enzymes involved in flavonoid synthesis, such as PAL, CHS, CHI, and ANS, increased with light intensity, with the highest activity at 200 μmol·m⁻²·s⁻¹. The expression of Ag3GT, a key gene for anthocyanin synthesis, reached its maximum at 300 μmol·m⁻²·s⁻¹, while AgFNS, a key gene for apigenin synthesis, reached its maximum at 200 μmol·m⁻²·s⁻¹. Overall, the anthocyanin content was highest at 300 μmol·m⁻²·s⁻¹, and the apigenin content was highest at 200 μmol·m⁻²·s⁻¹. The study concludes that a light intensity of 200 μmol·m⁻²·s⁻¹ is most favorable for celery growth and nutrient synthesis.This study investigates the effects of light intensity on the growth and flavonoid synthesis of celery (Apium graveolens L.) under hydroponic and full LED light conditions. Four light intensities (40, 100, 200, and 300 μmol·m⁻²·s⁻¹) were tested, with three harvest periods (15, 30, and 45 days after treatment). The results showed that increasing light intensity generally promoted celery growth, with the highest increases in plant height and aboveground biomass observed at 200 μmol·m⁻²·s⁻¹. Light intensity also significantly influenced the content of chlorophyll, carotenoids, total phenols, vitamin C, cellulose, total flavones, and apigenin in celery. The activities of key enzymes involved in flavonoid synthesis, such as PAL, CHS, CHI, and ANS, increased with light intensity, with the highest activity at 200 μmol·m⁻²·s⁻¹. The expression of Ag3GT, a key gene for anthocyanin synthesis, reached its maximum at 300 μmol·m⁻²·s⁻¹, while AgFNS, a key gene for apigenin synthesis, reached its maximum at 200 μmol·m⁻²·s⁻¹. Overall, the anthocyanin content was highest at 300 μmol·m⁻²·s⁻¹, and the apigenin content was highest at 200 μmol·m⁻²·s⁻¹. The study concludes that a light intensity of 200 μmol·m⁻²·s⁻¹ is most favorable for celery growth and nutrient synthesis.