Polymeric carbon nitride (p-C3N4) is thermodynamically feasible for photocatalytic overall water splitting. Element doping has been proved effective in enhancing the photocatalytic performance of p-C3N4. The effect of doping is usually interpreted from the angle of electronic structures by first-principles density functional theory (DFT) calculations. However, the information on electronic structures is insufficient for understanding and predicting the ultimate criterion of solar-to-hydrogen (STH) efficiency. Herein, we provided a DFT calculation method to investigate and predict the STH of VIA group elements doped p-C3N4 by calculating the efficiencies of both light absorption and carrier utilization. Particularly, significant efforts were made to calculating the energy barriers for surface hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to determine the carrier utilization efficiency. Moreover, the chemisorption energies of the reactant intermediates were calculated to quantify the intermediates affinity for HER and OER on the surface. Among the VIA elements, oxygen was discovered as the most effective dopant in promoting the STH because that oxygen-doped p-C3N4 has the lowest energy barriers for OER and the largest chemisorption energy for intermediates absorption. The calculation results highlight the importance of the surface reaction properties for efficient photocatalytic overall water splitting.