The planetary reducer is a common type of transmis-sion mechanism, which can provide high transmission accuracy and has been widely used, and it is usually required with high re-liability of transmission characteristics in practice. During the manufacturing and usage stages of planetary reducers, uncer-tainties are ubiquitous and wear is inevitable, which affect the transmission characteristics and the reliability of planetary redu-cers. In this paper, belief reliability modeling and analysis con-sidering multi-uncertainties and wear are proposed for planetary reducers. Firstly, based on the functional principle and the influ-ence of wear, the performance margin degradation model is es-tablished using the hysteresis error as the key performance parameter, where the degradation is mainly caused by the accu-mulated wear. After that, multi-source uncertainties are ana-lyzed and quantified separately, including manufacturing errors, uncertainties in operational and environmental conditions, and uncertainties in performance thresholds. Finally, the belief reliabi-lity model is established based on the performance margin de-gradation model. A case study of a planetary reducer is applied and the reliability sensitivity analysis is implemented to show the practicability of the proposed method. The results show that the proposed method can provide some suggestions to the design and manufacturing phases of the planetary reducer.