The establishment of a heterojunction is a crucial strategy to design highly effective nonnoble metal nanocatalysts for the photocatalytic nitrogen reduction reaction (PNRR). Heteropoly blues (r-POMs) can act as electron-transfer mediators in PNRR, but its agglomeration limits the further promotion of PNRR productivity. In this work, we construct a protonation-modified surface of N-vacancy g-C 3 N 4 (HV-C 3 N 4 ), achieving the high dispersion of r-POMs via the surface modification strategy. Enlightened by the synergy effect of the nitrogenase, r-POMs were anchored onto HV-C 3 N 4 nanosheets through an electrostatic self-assembly method for preparing r-POMs-based protonation-defective graphitic carbonitride (HV-C 3 N 4 /r-POMs). As an electron donor, r-PW 12 can match with the energy level of HV-C 3 N 4 to build a heterojunction. The electron redistribution of the heterojunction facilitates the optimization of the electronic structure for enhancing the performance of PNRR. HV-C 3 N 4 /r-PW 12 exhibits the best PNRR efficiency of 171.4 μmol L −1 h −1 , which is boosted by 94.39% (HV-C 3 N 4 ) and 86.98% (r-PW 12 ). The isotope 15 NH 4 + experiment proves that ammonia is derived from N 2 , not carbon nitride. This study opens up a crucial view to achieve the high dispersion of r-POMs nanoparticles and develop high-efficiency nonnoble metal photocatalysts for the PNRR.