To carry out the analysis and calculation of the transmission efficiency of tri-axial differential mechanism better,the theory of bond graph is applied to the analysis of power flow of the tri-axial differential mechanism utilizing the characteristic of power fluxion of bond graph.According to the transmission relationship and method of absolute speed,the bond graph model of the mechanism restricted by speed is established.The status of the internal power flow of the tri-axial differential mechanism,as well as the power flow of the conversion mechanism of the differentials,is got by simulation.The result of the simulation shows that the tri-axial differential mechanism belongs to the power conflux mechanism,and there is no circulated power and no possibility of self-locking.Based on the result of power flow,the transmission torque is analyzed,and the efficiency can be computed easily.This method is more systematic than the traditional method,and is convenient to be solved by computer.