Objective: To examine the association of sedentary behaviors with visual acuity among primary school children, and to provide a reference basis for the prevention and control of poor vision. Methods: Using stratified random cluster sampling method, a total of 4 969 students in grade 1-4 were selected from 5 primary schools in Guangzhou in 2017, and then were followed up in 2019. A validated questionnaire was used to collect data on children s sedentary behaviors, including doing homework, watching TV, playing computer/video games, and taking the bus/car between home and school. Visual acuity was measured in the baseline and the follow up using a 5 m standard logarithmic visual acuity chart. Multiple linear regression and Logistic regression were conducted to analyze the relationship between sedentary behaviors and visual changes. Results: The two year cumulative incidence of poor vision was 48.74%. Homework time was inversely associated with visual acuity of left eye( β=-0.012, 95%CI =-0.023- -0.002 ). Homework time was positively correlated with poor vision( OR =1.16, 95% CI =1.05-1.27). However, total sedentary time, screen time and bus/car time were not statistically associated with changes in visual acuity( P >0.05). Conclusion Increased homework time was associated with poorer vision in primary school students. Interventions to reduce the academic burden should be taken.