Abstract: Fatigue damage of pressing punch will destroy the uniformity of pressing stress and lead to excessive local stress. The dramatic rise of local stress will increase the chance of explosion. In order to improve the safety during press fitting, the effect of pressing punch structure parameters on fatigue life was studied by finite element analysis. It was found that the diameter of punch end D, platform height H and shoulder round angle R had an effect on fatigue life. The change rule of fatigue life with size change of punch end was worked out. Combined with the cannonball size and process requirements,optimized punch size was put forward to satisfy the fatigue life requirement.