Abstract: In order to resolve cracking problem appeared in the reliability test for the lower beam of the water tank, the body suspension point load was studied based on multi-body dynamics analysis and finite element analysis to analyze and verify the cracks taken place in the water tank lower beam. The results show that the abnormal cracking of the lower beam of the water tank is due to the stress concentration at the step of the lower beam. The maximum stress in the cracked position of the step is about 294.2 MPa with the material HC340LAD+Z and yield limit 340 MPa. However, when high-cycle running (more cycle times), fatigue damage can still occur. Considering the cause of cracking, a structural optimization program was proposed. Through road test verification, the optimized structure can meet the design requirements.