Abstract: In this paper,a lumped parameter model of proton exchange membrane fuel cell（PEMFC）hydrogen system for vehicles based on exhaust hydrogen recirculation and a two-dimensional CFD model of PEMFC stack were developed. Through transient simulation,the effect of hydrogen emissions on the operating characteristics of the hydrogen system and the PEMFC stack were studied under rated power conditions. The results show that the anode inlet pressure and flow rate fluctuate significantly during the emission process. The fluctuation amplitude and fluctuation time are directly related to the emission duration. The performance of the stack decreases during hydrogen emission,but it can quickly recover after the emission. The distribution of water and gas inside the anode is improved obviously after hydrogen emission.