Dimethyl sulfoxide (DMSO) based solution has been proven promising in preparing high-quality Cu₂ZnSn(S,Se)₄ (CZTSSe) absorbers. In the present work, H₂O is used as a simple and effective additive to optimize the film formation of DMSO based CZTS precursor. It is demonstrated that a small amount of H₂O introduction into the DMSO solution reduced the oxygen content in the CZTS precursor film, and thus limited the formation of SeO₂/Sn(O,Se)_x secondary phase in the final CZTSSe layer. As a result of improved morphology and carrier collection ability of absorber layer, the power conversion efficiency (PCE) is enhanced significantly from 7.45% for CZTSSe solar cell device prepared with pure DMSO precursor solution to 9.30% for that with an optimized DMSO/H₂O ratio. Applying the same preparing strategy to the Ag containing CZTSSe device, we finally achieved a PCE of 10.3% under 1-sun illumination.