Abstract: Analysis on the macroscopic and microscopic features of the fracture,reference to process of machining and assembling suggested that hydrogen spread into material during acid pickling and under the pulling stress after first assembling,the hydrogen accumulate at the position where the head and the rod connected and stress concentrated,so cracks produced and expanded till the stress stops.In second assembling,torque impelled cracks expands with high speed,then brittle fracture occurs.