Abstract: Based on the oceanographic survey in Yueqing Bay in May 2016, the zooplankton dominant species and their relations with the salinity were studied. At the same time the ecological threshold of Labidocera euchaeta abundance on the salinity gradient was analyzed with the Gauss model. The results showed that the species Labidocera euchaeta, as a dominant species in spring in Yueqing Bay, had dominant index of 0.061 and its abundance was significantly and negatively correlated with the salinity (p<0.01). Salinity is one of main factors affecting the horizontal distribution of the copepod. It is found that the optimal salinity is 21.36 with threshold ranges from 20.17 to 22.55. On the salinity gradient, the ecological threshold ranges from 18.99 to 23.73. In addition, the effect of salinity on the growth of Labidocera euchaeta and the role of ecological threshold are discussed for the monitoring and early warning of environment carrying capacity of Yueqing Bay.