Based on the data collected from seasonal comprehensive surveys of the marine ecosystem in Pishan waters, the middle of Zhejiang Province in November, 2015 and in February, May, and August, 2016, the crab community characteristics such as species composition, dominance species and diversity were studied. The similarity of community structure and the relation between community structure and hydrological environment factors were analyzed. A total of 19 crab species were identified, which belonged to 9 genera in 6 families. The dominant species were Portunus trituberculatus, Charybdis japonica, C. bimaculata and Eucrate crenata. The composition of crab species and dominant species showed clear seasonal changes. The number of crab species in summer was the greatest and the least was in winter. The distribution of crab species at various depth of water showed the following trend. The species at 5-10 m and 10-15 m depth of waters was major, the water depth of less than 5 m and 15-20 m was minor. The diversity index of summer showed high range of variaton, the diversity index of winter showed lower values of variation. Diversity index of crab at the 10-15 m depth was higher, the 5-10 m depth of waters was lower. Based on redundancy analysis, surface water temperature, surface and bottom salinity were the most important environmental factors affecting the change of the crab species composition and community structure characteristics.