Abstract: The fracture of the back trailing-arm of a vehicle was found after driving 21000 km in the test site. These back trailing-arms were broken for many times. The back trailing-arm was suffering complex loads such as tension, pressure and torsion during operation. Based on observing the appearance of the back trailing-arm, macro and micro appearance observation, metallurgical structure inspection, hardness, finite element analysis and the cause was found. The results showed that the fracture of the back trailing-arm was fatigue fracture. The fracture process of the back trailing-arm was as follows: Buckling deformation occurred at the middle mounting hole of the back trailing-arm under the action of compressive stress. When the back trailing-arm was suffering the tension again, the maximum stress was transferred to the middle mounting hole, and fatigue cracking occurred under the alternating load. The thickness of the back trailing-arm was appropriately increased to improve its stiffness. No similar fracture failure occurred in the back trailing-arm after the thickness was increased.