A-C:H films with low friction and good wear resistance have long been regarded as a potential space lubricating film. However, its superlubricity mechanism and failure process in vacuum still remains to be improved. To clarify its friction mechanism, here, we systematically investigated the tribological property of a typical a-C:H film under a high vacuum environment. The results show that the extremely low friction coefficient lasts 2700 cycles under a contact pressure of 930 MPa, and the entire friction process can be divided into three stages. The friction coefficient was first stable with a low value after a short period of running-in, then it underwent an evident fluctuation period and further decreased to an extremely low value (0.005) until it abruptly failed. The structural evolution of the a-C:H film on a sliding interface for different periods through the entire friction process was characterized and a dynamic friction mechanism was established. The self-mated (a-C:H/a-C:H) friction process and hydrogen passivation contributed to the decrease of the friction coefficient in the early stages of sliding. Then, the emission of hydrogen became evident under high local stress and more dangling bonds were exposed on the worn surface, which leads to the wild adhesion wear between the sliding surfaces. The alternated process between an old film and new film is in consonance with the fluctuation of friction coefficient. Afterwards, carbon onions with a closed spherical shell structure are spontaneously formed on the worn surface in the absence of the hydrogen passivation effect, which further reduce the friction coefficient to an extremely low value. This study provides guidance to the further design of a new generation of a-C:H films with a special structure that exhibits a better tribological performance in a vacuum environment.