Abstract: Based on the NCEP 1°×1° reanalysis data and the conventional observation data, the features and the physical mechanism of summer fog in Dalian were analyzed. The results showed that: the distribution of fog in Dalian presented the characteristics of most fog days in east part while least fog days in north part, and the fog days had an overall rising trend, reached a peak in 2005 or 2006. Heavy fog occurred most frequently in south and east part in June and July, but showed no monthly significant difference in the other parts. Heavy fog often took place from night to the next morning when continuous summer heavy fog occurred in Dalian, the subtropical high of the high altitude layer showed a zonal distribution, there were activities with weak slot and ridge in westerlies; the lower layer was in the front of the slot and behind subtropical high field, also in the southwest vapor transport field. Meanwhile, the ground was in the top or the forepart of low pressure field, which kept blowing weakly southerly or easterly. The existence of inversion temperature layer, weak divergency and vorticity, and the slightly vertical velocity vorticity field were favorable not only for the transportation and convergency of fog, but also for the formation and the maintenance of fog. In the study, forecast index of the climate characteristics of summer fog and the continuous fog days in Dalian was obtained, which could provide a reference for summer fog forecast in Dalian.