This study introduces robust screening methodology for the efficient design of delafossite CuM1-xM'xO2 solid-solution photocatalysts using band-structure engineering. The investigation not only reveals the formation rules for various CuM1-xM'xO2 solid solutions but also highlights the dependence on both lattice compatibility and thermodynamic stability. Moreover, the study uncovers the nonlinear relationship between composition and band gaps in these solid solutions, with the bowing coefficient determined by the substitution constituents. By optimizing the constituent elements of the conduction band edge and adjusting solubility, the band structure of CuM1-xM'xO2 samples can be fine-tuned to the visible light region. Among the examined photocatalysts, CuAl0.5Ga0.5O2 exhibits the highest H2 evolution rate by striking a balance between visible-light absorption and sufficient reduction potential, showing improvements of 28.8 and 6.9 times those of CuAlO2 and CuGaO2, respectively. Additionally, CuGa0.9In0.1O2 demonstrates enhanced electron migration and surpasses CuGaO2 in H2 evolution due to a reduction in the effective mass of photogenerated electrons. These findings emphasize the pivotal role of theoretical predictions in synthesizing CuM1-xM'xO2 solid solutions and underscore the importance of rational substitution constituents in optimizing light absorption, reduction potentials, and effective mass for efficient hydrogen production.