Abstract: A draw-in bolt of the lateral rod at the left inlet of an aircraft was found to have fractured during ground inspection. Macro and micro observation, metallographic examination, hardness testing and chemical composition analysis have been performed to find out the failure cause. The results show that the fracture of the draw-in bolt is fatigue facture, and the fracture was mainly caused by the coeffect of abnormal assembly pretightening load and the load in the inlet. Accordingly, three improvement suggestions were put forward:adjusting procedure, adding special tools, and perfecting process regulations. By monitoring the assembly process of the 20 aircrafts and collecting more than one year's flight feedback, no similar failure has occurred in the outfield.