Abstract: In view of problem of difficult fault diagnosis and prediction for diesel engine, the paper analyzed common faults and influencing factors of diesel engine, introduced method of extraction, analysis and processing of the failure data, and built a model of fault diagnosis and prediction based on probabilistic neural network. The simulation results show that the model can diagnose and forecast fault of complex mechanical system such as diesel engine and give diagnosis results accurately, and its accuracy rate of fault diagnosis and prediction is up to 94.84 percent.