Using the first principles method, we explored physical properties of hydrogenated g-C4N3. We found that the vacany site became the most stable H adsorption site. The electronic band structure and magnetic properties were strongly affected by the H adsorption concentration. At low H concentration, the g-C4N3 still preserved half metallic state. However, we observed a semiconducting band gap at 14.3% H concentration and the magnetic state disappeared. Despite this suppression of magnetic state, we found that the semiconducting H-g-C4N3 could be used for metal-free photocatalyst. The calculated band gap was 2.93 eV, and this corresponds to an optical wavelength of 423 nm. The conduction band (CB) edge was positioned at 1.54 eV higher in energy than H+/H2 level. This indicates that it has a strong ability to reduce H+ to H2. At the same time, the valence band (VB) edge position was 0.15 eV lower in energy than O2/H2O level. Overall, we propose that the hydrogenated g-C4N3 can be used as a potential metal-free photocatalytic material.