The Neogene loose sandstone has the characteristics of low diagenesis and poor cementation. Its structure is loose, and groundwater is easily enriched in this formation, making it the main seepage channel for groundwater. Based on the Xiangshan No.2 Tunnel, the application technology of the advanced support system of horizontal rotary jet grouting piles in the formation of mudstone interbedded with loose sandstone was systematically studied through on-site pile testing and numerical simulation. The secondary supplementary grouting technology was proposed to improve several technical deficiencies of the horizontal rotary jet grouting piles and solve the frequent problems of collapse of the face and sudden water and mud influx during the excavation process of Xiangshan No.2 Tunnel. A simplified calculation method for horizontal rotary jet grouting piles based on rod structure is proposed by refining the calculation model, which will facilitate the promotion and application of this support system.