In this study, a simple and reproducible two-step annealing process was demonstrated to prepare Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film with band-gap grading by selenization followed by post-sulfo-selenization (PSS) treatment. The effects of PSS treatment on the composition and electrical properties of the CZTSSe thin film were investigated. The crystal structure, surface morphology, surface roughness and element composition of the CZTSSe thin film were characterized by GIXRD, SEM, AFM and EDS tests. The composition of elements at different depths of the thin film was also investigated. It was confirmed that CZTSSe thin film with high sulfur element content on the surface and low at the bottom were obtained after PSS treatment. The electrical performance of the CZTSSe solar cells was measured by Hall test, EQE and J–V. The results showed that the power conversion efficiency (PCE) of the CZTSSe thin film without PSS treatment was only 7.62%, while the PCE of the CZTSSe thin film after PSS treatment increased to 10.11%.