Abstract: With the improvement of other key regions of China, the pollution problem on the Northern Slope of Tianshan Mountains has become increasingly prominent. In order to investigate the seasonal variation of ventilation capacity and its effect on local pollution, the seasonal variation of PM_2.5 pollution and meteorological conditions, along with their relationship, were analyzed by using multi-observation datasets synergistically. Meanwhile, the seasonal changes of ventilation were quantified by a regional air quality model. It was found that the PM_2.5 concentration in winter (from December to January) can reach 10.8 times that of spring to summer months (from May to August). The analysis of meteorological elements revealed that the seasonal variation of ventilation capacity was the primary external factor responsible for the large concentration difference in low atmosphere. The simulation results indicated that the ventilation capacity of some major cities in spring and summer was much stronger than that in winter, which could be 4.9 times the effects in winter. In terms of monthly variation, the strongest monthly mean ventilation was up to a factor of 5.6 of that in the weakest month. Although the Beijing-Tianjin-Hebei Region is at similar latitude as the Northern Slope of Tianshan Mountains, the difference in ventilation capacity between spring-summer and winter was less than 2.0. Thus, minimizing winter emissions through shifting the peak production among seasons will be an efficient way to improve air quality on the Northern Slope of Tianshan Mountains.