Based on the data from two oceanographic survey in the Sanmen bay during Dec.2007 and Jun.2008, this paper discussed the population density of Hoplocarida and Decapoda shrimps in winter and summer, species dominancy and diversity, involved in some impact factors such as background of topography and geomorphy, as well as hydrological features.Results showed that eurythermal and eurysaline shrimp species prevailed in Sanmen Bay.Shrimp specie number in summer was higher than the one in winter, as well as the weight and number densities, which were 93.32 kg/km²and 31.97 kg/km²,32.34×10³ind/km²and 19.32×10³ind/km² respectively.Parapenaeopsis tenella was the most important shrimp species in winter and Oratosquilla oratoria in summer according to index of relative significance(IRI).The species component in summer was more homogeneous than it in winter, so the weight and diversity index H'in winter(1.58, 1.33) were both lower than those in summer(1.78, 2.18).The water temperature outside the bay mouth is relatively higher in winter so more shrimp species are found here.While in summer, the bay mouth is the intersection by waters with different salinities, therefore shrimp species is rich.In addition, the Decapoda shrimp species in Sanmen Bay reach 25.00% to the total shrimp species in Zhejiang offshore, which means specie richness in bay is much lower comparing with that in Zhejiang offshore.