Environmental DNA (eDNA) is the general term of DNA released by organisms in ice, soil, air, water, sediment and other environments, which has been widely used in field investigation of aquatic biological resources in recent years. Platyrhina sinensis is mainly distributed in the western area of the North Pacific Ocean, and the coastal area of Zhoushan may be the northern limit of its distribution. In order to explore the feasibility of eDNA technology in the detection of P. sinensis, the qualitative and quantitative detection of P. sinensis in Zhoushan offshore was carried out based on eDNA technology at the same time. In this study, the COI gene sequences of P. sinensis and Platyrhina tangi were compared and analyzed. Primer Express 3.0 software was used to design specific primers and TaqMan probe of P. sinensis. Three fixed net survey stations A, B and C were designed in Zhujiajian offshore, Zhoushan, and samples of P. sinensis were collected regularly. Water samples (stations A, B1, B2, C1, C2 and D) were collected on December 19, 2017, April 14, 2018 and July 14, 2018, respectively, and eDNA droplet digital PCR detection was carried out. Significant difference analysis was conducted between eDNA detection results and water temperature and sampling sites. The results showed that: the concentrations of eDNA of P. sinensis were different at different stations and at different times. The effects of water sampling sites on eDNA concentration of P. sinensis were significant, and there were significant differences in eDNA at different sampling sites in different seasons. This study showed that eDNA detection was sensitive. Water temperature, water depth and substrate quality all affected the distribution of P. sinensis. The results of this study lay a foundation for eDNA tracking and monitoring of P. sinensis in other sea areas.