Through simulation experiments, four sampling methods (water collector method, diaphragm pump method, submersible pump method, and pipeline method) and four sampling volumes (1 L, 10 L, 100 L, and 1 000 L) were compared under summer and winter conditions. The results showed that the method of sampling impacts the analysis of phytoplankton.The density of phytoplankton collected by the four sampling methods in the summer was 1 180.00±20 cells/L, 971.00±20 cells/L, 964.00±20 cells/L, and 676.00±20 cells/L respectively, and 152.00±20 cells/L and 120.00±20 cells/L, 80.00±20 cells/L, 200.00±20 cells/L in winter. The phytoplankton density collected in the four sampling volumes in summer was 1 000.00±50 cells/L, 288.40±20 cells/L, 45.24±10 cells/L, 0.05±0.01 cells/L and 168.00±20 cells/L, 25.20±10 cells/L, 3.32±1 cells/L and 0.05±0.01 cells/L in winter. The results show that regardless of summer or winter, different sample methods and sampling volumes have significant differences in the number of phytoplankton in the ballast water. Therefore, the sampling method and sampling volume affect the analysis of phytoplankton inthe ship's ballast water. The systematic research is conducted to establish a unified and accurate sampling method for ship ballast water so as to provide technical support for ballast water port state inspection and to protect the port and marine ecological environment.