During the operation of a proton exchange membrane fuel cell (PEMFC), free radicals generated by PEMFC usually attack proton exchange membrane (PEM), causing the formation of crack or holes, which make the cell failure. Adding free radical scavenger material into PEM is regarded as a commonly modification method. Here, a Sn-doped CeO2 free radical scavenger was synthesized. By increasing the concentration of Ce3+, the free radical scavenging performance of CeO2 in PEMFC could be enhanced, which could avoid PEM thickness to decrease rapidly, and the durability of PEMFC could be improved. Density functional theory calculation and experimental results show that Sn-doping can cause lattice distortion of CeO2, reduce the formation energy of oxygen vacancy, and improve the formation of Ce3+ in CeO2. At the same time, the addition of Sn2+ can reduce Ce4+ in CeO2-Sn to form Ce3+, which is conducive to increasing the concentration of Ce3+ and thus improving the durability of PEM. The results of single cell test show that the voltage attenuation rate of the single cell assembled with CeO2-Sn-5% modified is the lowest (18%) and the power retention rate (56%) of the single cell is higher than that of other PEM after 70 h open circuit voltage decay test, which indicates that the sample has better durability.