In order to study the effect of diffusion layer porosity on the performance of proton exchange membrane fuel cell the performance curves gas concentration distribution and liquid water distribution of traditional parallel flow field and slope parallel flow field were numerically simulated and analyzed by using computational fluid dynamics commercial software ANSYS Fluent under the conditions of different diffusion layer porosity 0. 3 0. 5 and 0. 7 . The results showed that the performance difference of each case was small at high potential and the performance difference was large at medium and low potential. The greater the porosity of the diffusion layer the better the performance of proton exchange membrane fuel cell. When the porosity was 0. 3~0. 5 the maximum growth rate of current density was 9. 03%. When the porosity of the diffusion layer is high it is conducive for the reaction gas to pass through the diffusion layer which increases the oxygen concentration in the catalytic layer and promotes the electrochemical reaction in the fuel cell. With the increase of the pores in the diffusion layer it can more effectively promote the transmission of reaction gas. The higher the water content in the channel the more conducive to the discharge of liquid water. Compared with the traditional parallel flow field in the slope parallel flow field the cell performance is better the oxygen distribution is more uniform the gas flow rate in the flow channel is greater the drainage effect is better and the current density growth rate is the largest at a porosity of 0. 7 up to 28. 79%.