The wear particles were introduced into lubricating oil in the running-in process of the friction pair with relative movement in the heavy car axle, which leads to the decline of lubricating oil performance. The wear particles entered the contact surface of the friction pair with the lubricating oil, which leads to the aggravated wear. According to the running characteristics of heavy trucks, the used lubricating oils for decelerator under different driving mileages were sampled, and the wear particles in the oils were analyzed comprehensively for sizes, morphology and composition. The results show that the main wear particles in the lubricating oils are iron and copper debris, which are mainly derived from the friction pair of spherical shim (Copper) and gear pairs. The formation mechanism and producing process of wear particles were analyzed, which provide the theoretical basis of the design and manufacture of the moving friction pair in heavy truck decelerator.