Abstract: Automatic shutdown fault happened to a certaint ype of marine diesel engine during operation. By dissembling inspection, it was found that both the thrust plate screws and the thrust plate that fixed on the camshaft fractured, and the camshaft also fractured at the free end. Fracture reasons of the camshaft were analyzed by means of macro examination, chemical compositions analysis, mechanical properties test, fracture analysis and metallographic examination. The results show that traditional retracting processing made the transition radius of the camshaft was too small and the machining marks were obvious, so serious stress concentration formed at the chamfering position. At the same time, fatigue strength of the camshaf tdecreased because of lower hardness, serious banded ferrite structure and no surface treatment. Under the action of alternating stress, cracks formed from the stress concentration position and propagated in fatigue form. And then the fracture of the camshaft caused fracture of the thrust plate and its fixing screws, and finally resulted in shutdown of the diesel engine. Some feasible suggestions were put forward from the aspects of manufacture, repair and using management, fault analysis.