Abstract: After driving about 28 km, the connecting bolt between the lower swing arm and the frame broke. Through macro and micro observation and energy spectrum analysis of bolt fracture, the hardness and metallographic examination of materials near the fracture were carried out, the properties and causes of bolt fracture were analyzed. Then improvement measures were put forward. The results show that the fracture property of the bolt is low cycle fatigue fracture. The reason for the fracture is that the bolt is subject to alternating stress of two-way bending and large impact load due to insufficient preload and poor driving conditions during assembly. The assembly process is strictly controlled to avoid bolt loosening due to insufficient preload.