Graphic carbon nitride (g-C3N4) has recently attracted a great deal of attention for its application in solar energy conversion. The photocatalytic activities, however, suffer from fast electron-hole recombination and insufficient solar absorption of g-C3N4. Here, a modified g-C3N4 (CNF) photocatalyst, which was prepared by a simple hydrothermal treatment with dilute hydrofluoric acid (0.1 M HF), has achieved a 2.4-fold enhanced photo catalytic activity for hydrogen production under visible light irradiation compared to pristine g-C3N4. It is demonstrated that a structural distortion is introduced in the CNF sheets without the formation of C-F bonds after the hydrothermal treatment, which can effectively separate photogenerated electron-hole pairs. Interestingly, the g-C3N4 treated by oil bath reflux reaction with dilute hydrofluoric acid (CNFO) under the same reaction condition (concentration, temperature and duration) has no sign of structural distortion, and shows a decline in the photocatalytic H-2 evolution activity. This may be attributed to the enhanced localization mobility of pi electrons induced by the formation of C-F bonds after the oil bath treatment. These findings provide a better understanding on the effect of different fluorination treatments on the surface states and photocatalytic activities of g-C3N4-based materials. Additionally, this work presents a promising strategy to improve the photocatalytic H-2 evolution activity of g-C3N4 or related polymer semiconductors via microstructure engineering.