Aimed at the problem of oil leakage of the dynamic seal of an aviation air turbine starter in the process of design and development,the mechanical seal structure was decomposed and analyzed,and it was preliminarily determined that the leakage oil comes from the end face of the static ring and the dynamic ring of the friction pair.A detailed fault tree was compiled based on the structural characteristics of the dynamic seal and the service environment of the air turbine starter.The experimental research and simulation calculation of the medium temperature and friction pair temperature field of the dynamic seal were carried out,the end face of the friction pair was detected and analyzed,and the axial compression force required by the dynamic seal and the compression force provided by the spring were checked and calculated.It was concluded that the main reason for the oil leakage was the expansion of the auxiliary rubber ring in the high temperature oil,which leaded the increase of friction resistance.As a result,the elastic element could not provide enough pressing force for the friction pair.According to the research result,the preload of the dynamic seal wave spring was increased,and the sealing effect of the improved dynamic seal was verified by experiments.