The purpose of this study is to examine the association between hourly air pollution on hourly physical activity (PA) and sedentary behavior (SB) among college students in Beijing, China. The secondary aim was to examine such associations varied at specific time. A total of 340 participants were recruited from the Tsinghua University, in Beijing, China. Accelerometers provided PA measures, including moderate-to-vigorous physical activity (MVPA), walking steps, energy expenditure and sedentary time for 7 consecutive days. Corresponding air pollution data by the Beijing Municipal Ecological Environment Bureau in the closed site (Wan Liu site) in Tsinghua University were collected including average hourly air quality index (AQI) and PM_2.5 (μg/m³). Associations were estimated using linear individual fixed-effect regressions. We also conducted an air pollution risk perception survey among 2,307 freshmen (76.6% males) who were enrolled in Tsinghua in 2016, and the survey was done in May 22-26, 2017. A one level increase in hourly air quality index (AQI) was associated with an reduction in one-hour PA by 0.083 (95% confidence interval [CI] =-0.137, -0.029) minutes of MVPA, 8.8 (95% CI = -15.0, -2.6) walking steps, 0.65 (95% CI =-1.03, -0.27) kcals of energy expenditure. A 10μg/m³ increase in air pollution concentration in hourly PM_2.5 was associated with a reduction in one-hour PA by 0.021 (95% confidence interval [CI] =-0.033, -0.010) minutes of MVPA, 2.2 (95% CI = -3.5, -0.9) walking steps, 0.170 (95% CI =-0.250, -0.089) kcals of energy expenditure an increase in one-hour sedentary behavior 0.045 (0.005, 0.0845). At specific time, stronger negative associations of AQI and PM_2.5 air pollution with PA at 8 am, 4 pm, 5 pm and 7 pm. Similarly, stronger positive associations of one hour AQI and PM_2.5 air pollution with SB at 8 am, 9 am, 11 am, and 7 pm. A total of 94.9% participants (n = 2,235) responded “yes” to air pollution change activities in the survey, which may partially explain PA change. Air pollution may discourage physical activity and increases sedentary behavior among freshman students living in Beijing, China. This is preliminary study. The impact of air pollution on physical activity and sedentary behavior at a specific time may be different.