Nowadays, hydrothermal post treatment is a promising method for the modification of g-C3N4. This paper illustrates the post-treatment modification of g-C3N4 achieved by using a continuous hydrothermal system (CHS) with high heating rates (25–50 °C/s), short residence time (25.2–50.3 s), high reaction temperatures (250–300 °C) and high pressure (about 19 MPa). The continuous hydrothermal post treatment derived g-C3N4 (CHT-CN) prepared at 280 °C with 9 ml min−1 flow rate of g-C3N4 dispersion (CN-280/9) exhibits 5.6 times the photocatalytic H2-evolution activity of CN-30/9 under visible light irradiation (λ > 400 nm). Due to the exfoliation and shearing action of high-temperature and high-pressure water, the specific surface area of CN-280/9 reaches 65.8 m2 g−1, which is 4.3 times of CN-30/9 and therefore lead to an increase in reactive sites available for photocatalytic reactions. Moreover, the continuous hydrothermal post treatment results in the disruption of the conjugate structure of pristine g-C3N4 to build new channels, which allows more electrons to be transferred from bulk to surface to participate in the reduction reaction, and thus enhancing the H2-evolution activity. This work provides a valuable reference to potentially achieving the large-scale production of efficient photocatalysts based on the continuous high-temperature and high-pressure hydrothermal method.