The Ag-alloyed Cu1.9Ag0.1ZnSn(S,Se)4 (CAZTSSe) films were prepared by selenidation of Cu1.9Ag0.1ZnSnS4 (CAZTS) precursor films under various selenidation conditions. The effects of selenidation time and selenidation temperature on the composition, morphology, optical band gap (Eg) and device performance of CAZTSSe films were systematically studied. It is found that the selenidation temperature of 480 °C and selenidation time of 10 min provide a favorable selenidation environment for the growth of the film, which can promote grain growth and form a thick and smooth film. This has been strongly confirmed through relevant tests. In addition, the analysis of performance characterization of the films shows that the Eg of the films can be adjusted via adjusting the selenidation conditions, so as to improve the electrical properties. Finally, the CAZTSSe solar cell with a photoelectric conversion efficiency (PCE) of 5.94 % and an open circuit voltage (Voc) of 397 mV was successfully prepared at the selenidation temperature of 480 °C and selenidation time of 10 min.