The enclosed differential gear train, as a configuration of main reducer for coaxial helicopters, has compact structures and strong load-bearing capacity. However, complicated transfer paths have made it more difficult for fault diagnosis. To solve this problem, this paper focuses on analyzing contribution of the transfer paths of the enclosed differential main reducer, identifying the dominated paths for fault diagnosis. Firstly, based on lumped-parameter and condensation polymerization methods, a rigid-flexible coupling dynamic model of the main reducer is established. Secondly, the cracked sun gear and the position near the outer shaft bearing on the upper casing are set as the excitation source and target point, respectively. The transfer paths are determined based on mechanical structure. Finally, based on the power flow theory, contribution of each path is calculated and its changed laws with the input speed and load torque are analyzed. The results show that the contribution ranks of transfer paths changes with input speed, while contribution of path 4 is always the smallest and contribution of path3 is always lower than that of path2. Moreover, in the range of 2000 r/min and 8000 r/min, path2 is the dominated path way, with the expectation of the range of 4300 r/min and 6550r/min, where path 1 is the dominate pathway. Additionally, the contribution rank is not changed with the load torque. And contributions of path1, path2 and path4 reduce first and then keep unchanged, while the contribution of path3 increases first and then remains unchanged with the load torque increasing.