Abstract: The overtorque protection shaft is an important component of the gas turbine transmission system. In the course of cold blowing, the overtorque protection shaft of a gas turbine broke. To investigate the reason for the fracture, many methods are employed such as chemical composition test, inclusion analysis, strength and hardness verification, running condition analysis of the gas turbine, and macroscopic and microscopic appearance observing of the fracture. Results indicate that it is the inner defects that result in the fatigue fracture of the shaft.