Abstract: The electrode testing system is the cornerstone of water electrolysis electrode R&D. However, the differences of experimental configurations, electrochemical mechanisms, and non-interchangeable performance data for distinct testing systems, posing significant challenges for comparative analysis and optimized design of electrode performance. The difference of catalytic activity for electrolyzer, three-electrode and two-electrode systems based on proton exchange membrane water electrolysis(PEMWE) commercial catalyst is studied. Using the voltage decoupling model to discuss the effects of testing systems on activation, ohmic and concentration overpotentials. The results indicate that electrolyzer shows the lowest hydrogen production energy consumption, and the primary cause of performance differences stems from ohmic losses. Although electrolyzer assembly involves multiple device contact resistances, due to clamping force and high metal conductivity, the ohmic loss at 3 A/cm² is 54.3% and 37.2% of that in three-electrode and two-electrode systems, respectively. Solution resistance is the main reason for high ohmic losses in three/two-electrode systems.