At present, most of the engine fault diagnosis is specific to a particular type of failure, and the mapping relationship between the analysis object features and the faults is not clear, so that the extracted features have poor universality, resulting in slow development of engine fault diagnosis, and there is no a mature diagnosis system can be used in practical application. Vibration based engine fault diagnosis research trends are analyzed both at home and abroad. The generalized force at C.G.(the center of gravity of engine) is proposed to be the analysis object for engine fault diagnosis, and clear mapping relationship between the analysis object features and the faults is achieved. It is significant for developing new engine fault diagnosis methods.