A facile water-activated method is developed for preparing porous graphitic carbon nitride with carbon vacancies by co-pyrolysis of melamine and water at a relatively low temperature under an Ar atmosphere, resulting in an increased specific surface area and the efficient separation of photo-generated electrons and holes. As expected, the optimal catalyst exhibited a high H2O2 yield of 180 μM within 4 h and good cycling stability. The reported template-free method may provide a reference for the preparation of high-performance photocatalysts in a facile way.