Aiming at the characteristic of the transmission and development of fault in complex electro-mechanical products，a method of fault root causes tracing analysis for electro-mechanical products based on action unit was proposed. Firstly，the transfer process between action units was analyzed after the basic action unit was obtained by structuring decomposition of the whole machine function which was “function-motion-action”. Then a Bayesian network model was established based on action unit and fault phenomenon，and combined with reasoning algorithm of Bayesian networks to calculate the occurrence rate of each node and trace the maximum probability path to diagnose the fault action unit and its propagation process at the motion level. To find out the root cause of the failure of the action unit，the fault graph was used to describe the failure model of fault action unit and its transfer and development process. The trace diagnosis from the fault phenomenon to the fault cause can be realized through the diagnosis and analysis at action layer and in the interior of action unit of complex electro-mechanical products. Finally，the method of fault root causes tracing analysis based on action unit was used to deal with the root cause of the NC rotary table of some machining center，the results show that the proposed traceability diagnosis method can effectively diagnose the fault action unit and its propagation process，and reflect the transmission and development process of the internal components of the action unit from the angle of motion，which is convenient to find out the root cause of the fault and improve the efficiency of fault diagnosis for electro-mechanical products.