Irradiated medical ultra-high molecular-weight polyethylene (UHMWPE) must be validated through accelerated aging tests to meet the requirements for implantation into the human body. However, the free radical evolution mechanism of UHMWPE during accelerated aging remains obscure. We systematically analyzed the effectiveness of existing accelerated aging methods from the perspective of free radicals. The changes in the concentration and composition of the free radicals present in electron-beam-irradiated crosslinked UHMWPE during accelerated aging were evaluated using electron paramagnetic resonance spectroscopy. The evolution mechanisms of the free radicals during this process were analyzed; in addition, the mechanisms occurring in room-temperature air and under a 70 ℃ nitrogen atmosphere were analyzed and compared. The results revealed that accelerated aging evaluation methods had significant limitations. The evolution mechanism of the free radicals present on the surface of irradiated crosslinked UHMWPE as well as inside it in room-temperature air were significantly different from that observed in the accelerated aging environment. In addition, the generation rate and oxidation degree of the oxygen-induced free radicals in the two environments are different.