In present work, MoS₂/a-C composite films were synthesized by r.f. sputtering of MoS₂ and graphite targets, and the influence of C dopant on the structure, morphology, mechanical and the tribological properties of composite films in vacuum, ambient air and mutually alternated atmospheres were discussed in detail. Increasing the sputtering power applied on graphite target from 0 to 400 W, the film hardness increases significantly by one order of magnitude, and the optimized lubricant performance were obtained at 300 W. After sliding tests the morphology and structure of tribofilm and debris were investigated by using optical spectroscopy and Raman spectroscopy. The results reveal that the adaptive release of non-lubricant phase from tribofilm is a critical issue for determining the tribological performance of MoS₂/a-C composite films in different sliding atmospheres. The more exhaustive release of non-lubricant phase, the lower friction of rubbed counterparts can be obtained. More interestingly, the lubricant mechanism of tribofilms formed in alternated atmospheres is partially affected by their initial composition established at the beginning of sliding.