Abstract: To address the unclear mechanism of voltage variation under ultra-high-pressure operation, this study investigates the effect of pressure on the open-circuit voltage(OCV) of PEM electrolyzers. First, the relationship between OCV and pressure is derived based on the Nernst equation. Then, polarization curve tests are conducted at different temperatures(40 ℃, 60 ℃, 80 ℃) and anodic/cathodic pressures(0.1 MPa, 3.5 MPa, 35 MPa) to analyze the effect of electrolysis pressure on voltage variation. Finally, by comparing the experimental average voltage increment with the theoretical prediction, a temperature-dependent empirical correction coefficient (γ) is proposed, and a quadratic polynomial empirical model is established. The results show that increasing pressure shifts the polarization curve upward, and the voltage increment exhibits a logarithmic relationship with the pressure ratio; in contrast, increasing temperature lowers the overall voltage level. The correction model effectively reduced the deviation between experiment and theory, achieving a goodness of fit of R²=0.93195. A 200 hours constant-current operation at 35 MPa demonstrated stable electrolyzer performance, with an average hydrogen production energy consumption of 4.04 (kW·h)/Nm³; moreover, high-pressure operation significantly reduced the degradation rate of contact resistance. These findings elucidate the mechanism of OCV response under ultra-high-pressure operation and provide theoretical and experimental reference for the operating condition design and efficiency improvement of high-pressure direct hydrogen production systems.