For the analysis of the main sources of phosphate in the ESC during summer, we choose the area between 120.93 ˚ E-125.9 ˚ E and 26.08˚ N-32.35 ˚ N as research sites . The samples were all from the voyage of the research ship (Xiangyanghong 18). Surface seawater was collected at 49 stations for δ18Op analysis by a conductivity-temperature-depth profiler. Then we utilized δ18Op to trace the source of phosphate in the ESC. The δ18Op values in seawater are affected by water masses with different phosphate concentrations.We need the two-component mixing model for the analysis of water masses that control phosphate.In this research, atmospheric deposition and adjacent sea transportation were used as terrestrial and seawater end-members respectively to construct the δ18Op end-member mixing model. In order to quantify the source of phosphate in the sea, a Bayesian isotope mixing model run in the R software package (Stable Isotope Analysis in R, SIAR) was selected for analysis.