For the cracking problem of cylinder head in the test process, the stress state of cylinder head was evaluated from multiple dimensions such as cooling fluid field, temperature field, stress, deformation and fatigue safety factor by means of simulation method in order to explore the load-acting mechanism and fatigue cracking mechanism leading to the failure. The results show that the area with the lowest fatigue safety factor in the simulated assessment model is consistent with the cracking position in the actual using process and it cracks due to the fatigue cyclic cracking load composed of temperature and gas force load.