Objective:To explore the effect of physical activity trajectories on cognitive function in middle-aged and older adults.Methods:Based on data from the China Health and Retirement Longitudinal Study (CHARLS) from 2011 to 2020, 809 participants aged ≥45 were selected. General data, physical activity levels, and cognitive function were collected. The group-based trajectory model (GBTM) was used to establish the physical activity trajectory model, and multiple linear regression was applied to investigate factors affecting cognitive function in middle-aged and older adults and to analyze the relationship between physical activity trajectories and cognitive function.Results:The cognitive function score for the 809 middle-aged and older adults was (11.71±4.60), and the logarithmic metabolic equivalent of physical activity was (8.38±1.06). Based on GBTM, four physical activity trajectories were identified: stable group, slow-increase group, rapid-decrease group, and stable-decrease group. Multiple linear regression analysis showed that the stable and slow-increase activity trajectories, gender, age, marital status, and retirement status were significant factors influencing cognitive function in middle-aged and older adults ( P<0.01) . Conclusions:Physical activity trajectories are associated with cognitive function in middle-aged and older adults. Maintaining stable or slowly increasing physical activity is beneficial for cognitive health. Nurses should design targeted interventions based on factors influencing cognitive function.