The service life of the planetary reducer in a hub drive system largely depends on the system reliability design. Aimed at the problems of lower credibility and inadequate depth in the reliability allocation, based on improved FMECA formulas and order statistics theory, an advanced reliability allocation method of the planetary reducer is proposed. The basic characteristics of the planetary reducer are analyzed, and FMECA formulas with the positive failure correlation modifying operator are presented, which indicate that the subsystem complexity and the failure occurrence change in the same direction as the failure correlation degree, respectively. Simultaneously, based on the Gumbel Copula function and order statistics theory, the reliability allocation model is established to assign reliability design indices to parts of the planetary reducer. By comparing the subsystem reliability allocation weights obtained by different methods, results show that the allocation result of the proposed method is more credible. It will be helpful to accomplish the deeper reliability allocation and improve the service life of the planetary reducer.