Abstract: To investigate the effect of inlet gas relative humidity on the performance of proton exchange membrane fuel cell stack under variable load conditions, the dynamic cycling condition was designed with two step conditions of current densities ranging from 0.10 to 0.35 A/cm~2 and from 0.10 to 1.00 A/cm~2, and steady state condition of current densities of 0.10, 0.35 and 1.00 A/cm~2. Five sets of tests with humidity levels of 20%, 30%, 50%, 70% and 80% were performed. The results show that the sensitivity of the stack to humidity is different at different current densities under the constant current condition, and humidity has great impact on the stack performance at high current density. The stack voltage appears to be downstroke and upstroke in the variable load process, and the amount of voltage downstroke in the loading process is related to the variable load amplitude and humidity, while the voltage consistency of single cell of the stack is deteriorated and related to the inlet gas relative humidity.