A　series　of　In2S3　thin　films　were　obtained　via　chemical　bath　deposition　method　on　the　FTO　glass　in　the　acidic　solution,　with　citric　acid　as　the　complexing　agent.　XRD,　SEM,　UV　were　adopted　to　characterize　phase　structure,　morphology　and　optical　transmission　of　the　thin　films.　The　results　indicated　that　all　the　thin　films　were　cube　structured　beta-In2S3,　with　uniform,　network-like　morphology.　Transmission　of　films　was　step-down　with　the　thickness　of　films　increasing.　Band　gaps　were　calculated　between　2.5　and　2.6　eV.　The　effect　of　concentration　of　the　complexing　agent　on　the　thin　films　formation　mechanism　was　investigated.　The　results　showed　that　when　the　concentration　of　citric　acid　was　lower,　control　step　of　the　whole　reaction　system　was　the　complexation　balance　of　citric　acid　ions　and　metal　indium　ions,　and　when　higher,　the　key　reaction　was　that　thioacetamide　decomposed　to　form　sulfide　ion　under　acidic　condition.