In this work, SnS-SnSe composite thin films are synthesized by plused laser deposition method. The initial discharge capacity of SnS-SnSe composite film is 1225.8 mAh g-1. The reaction mechanism of the composite with Li has been investigated. It is revealed that SnS-SnSe composite thin film firstly reacted with Li during discharge to form Li2S、Li2Se and Sn, then Sn reacted with Li to form LixSn alloy. During charge process, LixSn firstly decomposed to form elementary Sn, then Sn catalyzed Li2S and Li2Se to decompose to reform SnS-SnSe composite thin film. Li2S and Li2Se produced during discharge process act as buffer material which might release stress caused by volume change, thus enhancing cycling performance of the component film. Additionally, SnS-SnSe composite thin film possesses more grain boundaries than single SnS or SnSe thin film, which could effectively enhance electrochemical activity and contribute to the capacity increment.