Among various deposition techniques to deposit (CZTSSe) thin films, solution-based processes have attracted considerable attention because of their potentially low cost. Most of the reported solution-based methods are based on nonaqueous solvents, such as hydrazine, and organic solvents. We report the deposition of CZTSSe thin films and fabrication of CZTSSe solar cells by a water-based, solution-processed method followed by Se vapor annealing. The effects of Se vapor feeding time on the properties of CZTSSe thin films and the performance of CZTSSe solar cells are investigated. The ratio of can be tuned by changing the Se vapor feeding time. Our results indicate that extending the Se vapor feeding time increases the band gap, slightly increases the lattice constant, and significantly improves the morphologies of the CZTSSe thin films. A remarkable enhancement in the performance was observed from the CZTSSe solar cells annealed with a longer Se vapor feeding time compared with those without Se feeding.