Crooked shaft grinder is the key machining equipment of engine Crooked shaft, as the principal axis system of grinder head failure occasionally, it influences the cutting precision greatly. So it has significance to research on failure diagnosis method for engine crooked shaft grinding equipment. The method, which diagnoses the causes and degree of failure, based on fuzzy logic and recognition theory is presented for the principal axis system of crooked shaft grinder. The mathematics model of fuzzy recognition and the conditional distribution function of fuzzy relationship between failure and symptom are given and the method of determining the function is simple and practicable. The method that is got through fuzzy reasoning and fuzzy recognition can be used not only to diagnose failure cause but also to determine failure level. The vibratory diagnosis for the principal axis system of crooked shaft grinder shows that it possessed a good rationality as well as availability.