In the construction of a tunnel in sandy rock mass on LanzhouChongqing highspeed railway, advance reinforcement is made by means of horizontal jet grouting. The distribution of the pressure of the sandy rock mass and the loading characteristics of the primary support structure are analyzed by means of finite element numerical simulation and on basis of the monitoring data of the rock mass pressure and steel arch stress. The study results show that the safety degree of the primary support structure of the tunnel prereinforced by horizontal jet grouting can meet relevant requirements and has some margin. The paper can provide theoretical reference for the optimization design and construction of tunnels in sandy rock mass in the future.