Solid solutions between SrTaO₂N and CaTaO₂N, i.e. Sr_1-xCa_xTaO₂N (0.0 ≤ x ≤ 1.0) were synthesized and investigated as photocatalysts for water oxidation and reduction. A wealth of important parameters such as Ta-(O, N) bond length, Ta-(O, N)-Ta bond angle, band gap and band edge positions can be successively tuned by varying Sr/Ca molar ratios at A site. These solid solutions work well as photocatalysts to drive water splitting reactions under visible light illumination which clearly outperform their parent compounds SrTaO₂N and CaTaO₂N. Apparent quantum efficiency as high as 1.67% at 420 ± 20 nm was achieved on Sr_0.4Ca_0.6TaO₂N (x = 0.6) which stands as the highest one reported for SrTaO₂N and CaTaO₂N to date. This simple approach by cation control at A site serves as a useful tool to tailor photocatalytic properties of perovskite oxynitrides, opening new playgrounds for the design and development of these tempting compounds as efficient photocatalysts.