Abstract: The liquid water produced by an electrochemical reaction at the cathode of a proton exchange membrane fuel cell blocks the pores in the gas diffusion layer, resulting in “water flooding.” At the same time, membrane dehydration leads to serious ohmic polarization. Discharging liquid water from the stack as soon as possible to ensure the wetting of the proton exchange membrane is a key problem. A membrane humidifier is a key component of a proton exchange membrane fuel cell system for water and thermal management. By considering coupling with the working conditions of a fuel cell, systematic sensitivity simulation analysis of the operating and geometric parameters of the membrane humidifier was carried out. The steady-state mathematical model of the membrane humidifier was established based on Matlab/Simulink. The influences of the inlet mass flow rate, temperature, and pressure, membrane thickness and area on heat transfer, water transfer, relative humidity, and water transfer rate of the membrane humidifier on the wet and dry sides were analyzed. The main conclusions are as follows: Improving the inlet mass flow rate can effectively improve the heat transfer and water transfer quantity, yet reduces the water transfer rate and the relative humidity at the drying side outlet. The increase in temperature on both dry and wet sides can improve the diffusion coefficient and transfer capacity of water in the membrane; however, high temperature significantly increases the saturation pressure of water vapor, reduce water activity, and then reduce the water content of the membrane, which is not conducive for water transfer. The change in pressure has little effect on heat transfer; however, an increase in the total pressure reduces the inlet moisture content and water transfer capacity while increasing the water transfer rate. A larger membrane area and a lower membrane thickness can improve the film moisture transfer and water transfer rates, which can effectively improve the membrane humidifier and fuel cell system hydrothermal management performance.