Engine cylinder gasket is not prone to leakage under stable conditions, but leakage and cracking often occur during alternating tests such as thermal shock. In order to study the failure of cylinder gasket during alternating tests, this paper establishes a spring unit model of gasket through finite element method, and simulates the transient temperature field results of the thermal shock test cycle. The problem of high cycle under low cycle process was studied, and the changes of line load and gap movement of each position of the gasket during the whole cycle were calculated, and the differences between the transient process and the steady process were obtained. It can be seen from the results that the transient analysis results are basically consistent with the test results, the minimum line load of the gasket is 6.3 N/mm, and the maximum gap movement is 42 μm. By increasing the stiffness of the half beads at the failure position, after optimization, the minimum line load of the gasket is 10.2 N/mm, an increase of 62%, and the maximum gap movement is reduced to 36.8 μm, a decrease of 12.4%. The effect is obvious, and the problem of leakage and cracking of the gasket is solved.