Some haze-fog events in Asia have been attributed to tropical cyclone activity; however, uncertainty exists regarding the relationship between the influence of tropical cyclones and the occurrence of haze-fog events. In this study, the statistical relationship between tropical cyclones in the Northwest Pacific and the haze-fog events in northern China during summers from 2001 to 2012 were analyzed. It was found that 30.5% of regional haze-fog events were related to tropical cyclones. The influence of tropical cyclones on haze-fog events was analyzed and compared based on classification of tropical cyclones by position and path. The results showed that tropical cyclones can form advantageous conditions for regional haze-fog events through dynamic and thermal processes, such as strengthening the downdraft, and increasing relative humidity and stability. The dynamic influence was dominant when the distance between a tropical cyclone and northern China was larger than the range of tropical cyclone airflow, and the contribution of thermal influence increased as the distance decreased. Furthermore, the surrounding circulations, such as the Northwest Pacific subtropical high and the westerly trough, also contributed to the regional haze-fog events. Their position, intensity and collocation with tropical cyclones could be the determining factors for haze-fog occurrence. This study illuminates the primary mechanisms of the combined effect of tropical cyclones and the surrounding circulations on regional air quality, which could improve forecasts of summer regional haze-fog events in northern China.