Abstract: Three 35Ni4Cr2MoA steel connecting bolts of titanium alloy inner pressing plate of fixed tail reducer of a helicopter were fractured during the test run. The fracture causes of the connecting bolts were analyzed by means of macro observation, fracture analysis, metallographic examination, mechanical property test and hydrogen content test. The results show that the failure mode of the connecting bolts was fatigue fracture. The fatigue crack source was formed by the local extrusion damage between the hole wall of the pressing plate and the bolt rod. During the test run, the cracks continued to expand, resulted in the fracture of the connecting bolts.