Based on the analysis on diagnosis knowledge of cold start fault for diesel engine, the running state parameters of the process of cold start in the diesel engine were processed by using clear sets to construct a fuzzy set. Some abnormities about the process of cold start in the diesel engine were diagnosed by using fuzzy inference algorithm, and an abnormity diagnosis method about the process of cold start in the diesel engine was put forward based on fuzzy logic inference. Application results reveal that the probability about abnormity of the start difficult was 0.752. Therefore, oil temperature and inflow manifold temperature should be checked. Application effects show that the abnormity diagnosis method about the process of cold start in the diesel engine is of high accuracy and large practicality.