Conjugated structure modification is an effective strategy for improving the photocatalytic activity of graphitic carbon nitride (g-C₃N₄). Herein, we introduce p-π conjugated to g-C₃N₄ by surface grafting of hydroxyl (-OH) groups through a facile hydrothermal process to improve the photo-carriers generation and separation. Experiments and DFT calculations indicate that the hydrothermal treatment decorated –OH groups onto the C sites of g-C₃N₄ and formed p-π conjugated structure between the 2p orbitals of the O and the π orbitals of the tri-s-triazine unit. The extended p-π electrons delocalization of g-C₃N₄ increases the concentration of the carriers, and the –OH groups serve as electrons transport channels facilitating the separation of the carriers. Consequently, –OH grafted g-C₃N₄ with hydrothermal treated at 120 °C exhibits the highest and recyclable H₂ evolution rates of 2.3 mmol h⁻¹ g⁻¹ under simulated sunlight irradiation, which outperform pristine g-C₃N₄ 1.42 times. This work proposes a new mechanism of the p-π conjugate of –OH decorated g-C₃N₄ for improving charge carriers formation and separation, and provides a mild strategy to engineer the other conjugated molecular structures.