In order to solve the problem of cracking of the cab rear suspension in the process of endurance road test, the fatigue analysis based on the traditional quasi-static method fatigue analysis and torsion fatigue analysis are carried out, but the cracking problem of the rear suspension cannot be reproduced by this way. Through modal analysis, it is found that the first modal frequency of the rear suspension is greatly reduced due to equipped with the battery frame. This first modal frequency is coupled with road excitation, which result in resonate of the whole assembly. So the cracking is owing to the vibration fatigue damage. Therefore, a fatigue analysis method for rear suspension assembly based on modal superposition method is developed to accurately reproduce the fatigue cracking problem. Finally, the rear suspension structure is improved based on modal superposition method, and the improved structure is proved to be accurate for vibration fatigue prediction by road test.