In view of the low accuracy of fracture network modeling in the numerical simulation of grouting in fractured rock mass, and the large difference between the simulation results of fracture grouting and the corresponding actual project, a numerical simulation method of three-dimensional(3D) random fracture network grouting considering underground pressure gradient is proposed. By optimizing the 3D fracture network generated by Latin hypercube sampling method, the modeling accuracy of underground 3D fractured rock mass and the grouting simulation calculation speed were improved. Through real-time observation and statistics of the changes of grout pressure field and grout flow in the fracture network, the fine simulation of the grouting process was achieved. Taking a grouting hole in the curtain grouting area of a hydropower project as an example, the simulated grouting volume and grouting time were compared with the actual grouting volume and grouting time. The results show that compared with the traditional numerical simulation of grouting in fractured rock mass with segmented fixed pressure, the simulation method considering the underground pressure gradient proposed in this paper is smoother in the pressure field during the slurry diffusion process, and the slurry diffusion distance is farther. The final grouting curtain is more complete, and the grouting volume is close to the real engineering grouting volume. The research results can provide reference basis for grouting simulation with multiple cracks and prediction of grouting effectiveness in practical engineering.