Abstract: By chemistry composition,microstructure,hardness,fracture and EDX analyse,the broken bolt had been analyzed. The results showed that the undercut slot caused stress concentrention because of bad machinning in thread root,there existed inclusions in material,and other defects were also observed. Such as poor assemble quality and damaged parts. The flexible diaphragm coupling bolts were used in the condition of great load and frequently start. Once the working stress exceeded the allowed stress line of bolts,the fatigue cracks might propagate and lead to fracture,finally caused the couple parts damage. How to design,make,install and use,was proposed.