This paper presents a method to identify the root cause of the axial vibration of crankshafts for high speed diesel engines based on an auto-regressive and moving average model and the analytic hierarchy process. Through determining initial moving average variables and measuring axial/bending/torsional vibrations of a crankshaft at the free-end of a four-cylinder diesel engine, auto-regressive spectrum analysis is applied to the measured vibration signal. In an investigation of the root cause of the vibration, the hierarchy tree and judgment matrix are given to identify the main vibration root causes. The results show that the axial vibration of the crankshaft is mainly coupled and excited by the bending vibration at high speeds. But at low speeds, the axial vibration in some frequencies is coupled and excited primarily by the torsional vibration. Through investigation of the root cause of the axial vibration of the engine crankshafts, calculation accuracy of the vibration can be improved significantly.