Ionomer is an important part of catalytic layer in Proton exchange membrane fuel cell.membrane electrode. Its main role in the catalytic layer is to conduct protons. In this paper, the effect of ionomer on the Pt/C catalyst was studied using the rotating disk electrode (RDE) under two modes, which simulated the real operating environment (mode 1) and simulated the start-stop environment in the fuel cell (mode 2). The structural changes of Pt/C catalyst after the durability test were analyzed by identical location transmission electron microscopy (IL-TEM). The results showed that ionomer could improve the durability of Pt/C catalysts. After the durability test of mode 1, adding ionomer can reduce the △E1/2 of oxygen reduction reaction from 23mV to 11mV. There is not carbon corrosion, and the growth of Pt particles was the main reason for the degradation of Pt/C catalyst. Ionomer delayed the decrease of electrochemical specific surface area (ECSA) of Pt/C catalyst, which was beneficial to the maintenance of Pt activity. After the durability test of mode 2, adding ionomer can reduce the △E1/2 of oxygen reduction reaction from 25mV to 5mV. In addition to the growth of platinum particles, carbon corrosion occurred in the catalyst layer. The presence of ionomer can also maintain the activity of Pt, and the growth of platinum particles is mainly due to carbon corrosion. The growth of platinum particles and carbon corrosion can be clearly seen by IL-TEM. The corrosion of carbon resulted in the loss and agglomeration of platinum. The average particle size of platinum in sample 1 increased from 2.7nm to 3.76nm, while that of sample 2 increased from 2.44nm to 4.19nm.