Van der Waals (VDW) heterojunctions in a 2D/2D contact provide the highest area for the separation and transfer of charge carriers. In this work, a top-down strategy with a gas erosion process was employed to fabricate a 2D/2D carbon nitride VDW heterojunction in carbon nitride (g-C3N4) with carbon-rich carbon nitride. The created 2D semiconducting channel in the VDW structure exhibits enhanced electric field exposure and radiation absorption, which facilitates the separation of the charge carriers and their mobility. Consequently, compared with bulk g-C3N4 and its nanosheets, the photocatalytic performance of the fabricated carbon nitride VDW heterojunction in the water splitting reaction to hydrogen is improved by 8.6 and 3.3 times, respectively, while maintaining satisfactory photo-stability. Mechanistically, the finite element method (FEM) was employed to evaluate and clarify the contributions of the formation of VDW heterojunction to enhanced photocatalysis, in agreement quantitatively with experimental ones. This study provides a new and effective strategy for the modification and more insights to performance improvement on polymeric semiconductors in photocatalysis and energy conversion.