Limited by the wide band gap of h-BN, the application of h-BN in photocatalysis is extremely challenging. In this paper, a strategy of co-doping monolayer h-BN with sulphur and iron was proposed to enhance the stability and catalytic activity of photocatalysts. Firstly, based on density functional theory (DFT), several models of monolayer h-BN nanosheets with iron decoration was created and optimized. Secondly, the structural, electronic, and optical properties were investigated in this paper, and the calculations proves that the introduction of impurity energy levels mainly composed of Fe 3d orbitals is the key to solving the wide bandgap of boron nitride. The HOMO-LUMO analysis shows S atoms participate in the redistribution of electrons, which inhibits the recombination of electron-hole pairs. This work verified the higher photocatalytic ability and wider light absorption range of monolayer h-BN co-doped with sulfur and iron. This paper has pointed a promising path for the modification of h-BN in the direction of photocatalysis.