Modifying the structure of photocatalyst to tune its electronic and physicochemical properties is an effective approach for efficient photocatalysis. Herein, we modify the structure of g-C3N4 by partially replacing the corner site C atom with P heteroatom via a simple thermal phosphorization method. Comparing with the pristine g-C3N4, the g-C3N4 after phosphorization shows an improved photocatalytic hydrogen activity, which is attributed to the narrowed bandgap and the upshifted conduction band edge, enhanced separation and transfer of photogenerated charges, and highly hydrophilic surface absorbing reactant molecule for photocatalysis. Various experimental characterizations are conducted to systemically explore the underlying essential mechanism behind the superior photocatalytic performance of g-C3N4 with phosphorization treatment. This work provides a simple approach to modifying the structure of g-C3N4, which could be applied to other semiconductors for designing photocatalyst with unique structure and enhanced activity.