To analyze the mechanism of a surface piercing propeller cup section′s water exit and entry, the cup section of the 841-B surface piercing propeller was modeled, and a reliable numerical method was established by solving the RANS equation to simulate the water entry process of the cup section by combining the standard k-Ε turbulence model, VOF method, and overlapping grid technique. On this basis, the mechanism of the section′s water exit and entry was studied, and the free liquid surface form, ventilation cavity, flow field form, and mechanical characteristics of the cup section under different advance coefficients were analyzed. The results showed that the water exit and entry process of the cup section was unsteady, with strong nonlinear change of the free liquid surface. During water entry, the pressure dropped and inhaled air to form a ventilation cavity. When the next section entered water, the liquid surface was more affected. Increasing the advance coefficient inhibited this ventilation phenomenon. In addition, the change in the ventilation cavity affected the mechanical characteristics of the surface piercing propeller. © 2024 Editorial Board of Journal of Harbin Engineering. All rights reserved.