For the oil leakage problems near the joint of engine cylinder cover, engine cover and timing cover during the durability test, the finite element model of engine was built to simulate the engine cold condition. Using the numerical analysis method, the stress of joint surface and the deformation of each component near the oil leakage point were analyzed. In the end, it was determined that the reason of oil leakage was the insufficient stiffness of bolt seat. The oil leakage problems were solved after improving the stiffness of bolt seat.