Based on ceilometers inversion data and simulation data, the dataset of mixed layer thickness in Tianjin were constructed, the data of PM2.5 mass concentration and visibility in Tianjin during 2009-2015 were collected, and the threshold and regularity of mixed layer thickness in heavy pollution weather in Tianjin were researched in this paper. The results show that the thickness of mixed layer kept increasing during 2000-2015, having a strong consistency with the increase of nocturnal boundary layer height and the increase of inversion base observed by meteorological tower. Statistics show that PM2.5 mass concentration has exponential relationship with the thickness of mixed layer. The lower the mixed layer thickness, the higher the PM2.5 mass concentration. The thresholds of mixed layer thickness can be 200 m, 400 m, 600 m and 800 m, which can be used as limits to determine the vertical diffusion of atmospheric pollution. When the thickness of mixed layer is less than 200 m, probability of heavy pollution weather is 52%, and probability of moderate haze is 46%. The negative correlation between PM2.5 average mass concentration and mixed layer thickness is not applicable to all pollution processes. For the transport of air pollution, the impact is from high altitude to ground surface. In the initial stage of pollution, the increase of mixed layer thickness is beneficial to pollutant transmission, causing the nearsurface PM2.5 mass concentration to increase, which need to be considered carefully when the threshold of mixed layer thickness is used.