BCN (boron carbon nitride) nanosheets are promising photocatalyst materials for solar fuel production by visible light-driven water splitting and CO 2 reduction due to their tunable band gap and unique properties. C-doping into h-BN by thermal annealing makes possible the preparation of BCN nanosheets with photocatalytic activity under visible light irradiation, but it generally requires a very high temperature (>1250 °C) from the thermodynamic viewpoint. Here, we report a new method to prepare BCN nanosheets with visible light-photocatalytic activity at lower annealing temperature (1000 °C) than equilibrium by adding alkaline earth metal compounds. BCN nanosheets formed in borate melt show a clear layered structure, tunable bandgap and photocatalytic activity for water splitting and CO 2 reduction under visible light illumination. This provides a direction for doping other elements into h-BN at low annealing temperature by alkaline earth metal borates.