In this study, an efficient porous g-C3N4 nanosheet with N defect was successfully fabricated via a facile co-thermolysis method. The porous g-C3N4 nanosheet showed 96% degradation efficiency towards Eosin-Y within 100 min, which was about 1.48-fold higher than that of pure bulk g-C3N4. In addition, the modified porous g-C3N4 nanosheet exhibited the hydrogen evolution performance with the activity of 450 μmol with 3 h, which was 5.29-fold higher than that of pure bulk g-C3N4. The elevated photocatalytic activity was attributed to the synergism of porous nanosheet and N defect, which improved light harvesting and accelerated separation of photo-induced e– and h+, greatly assisting in organic pollutants removal and hydrogen evolution. Furthermore, trapping experiments demonstrated that the holes and superoxide radicals were the main active species in the photocatalytic degradation of Eosin-Y solution.