By using the first-principles calculation based on density functional theory, we propose some approaches to improving the efficiency for the photocatalyst Ag2ZnSnS4 from a theoretical aspect. Comparing its band edge positions with those of other similar compounds, we find that Cu, Ge codoping can adjust both the band gaps and band edge positions of Ag2ZnSnS4 at the same time, which can optimize its band structure for water splitting. In addition, Ag2ZnSnS4 has a type-Ⅱ band offset with another photocatalyst CuGaSe2. Preparation of its homojunction can also improve their efficiencies of photocatalysis hydrolyzation.