Abstract: The Tertiary semi-diagenetic rocks are characterized by weak cementation and susceptibility to water softening, which can easily trigger underground geological disasters such as water inrushes, mud outbursts, and collapses during tunnel construction. Focusing on the Wangjiazhai tunnel of the Linqing Expressway in Yunnan Province, and considering the site’s geological conditions and relevant engineering case studies, this research employs horizontal high-pressure rotary jet grouting piles to pre-emptively strengthen the weak surrounding rock. Through a combination of theoretical analysis of foundation beams, numerical simulations, and field monitoring methods, this study investigates the reinforcement mechanism of horizontal high-pressure jet grouting piles in Tertiary semi-diagenetic formations and examines the impact of varying pile parameters on the effectiveness of surrounding rock reinforcement, offering a suggested range for these parameters to serve as a reference for similar projects. The findings indicate that the distribution of shear force and bending moment in both analytical and numerical solutions aligns with those in the unsupported section, where the pile body experiences the highest bending moments and shear forces, rendering it most susceptible to fracture failure. At a surrounding rock water pressure of 300kPa, the maximum tensile force within the rotary jet pile reaches 598.21kPa, nearing the pile’s ultimate tensile strength. When the water pressure is below 300kPa, the horizontal rotary jet pile can effectively leverage the combined effects of beam and arch principles, significantly enhancing both the arch action and the water-blocking effect. This allows the surrounding rock pressure to be redistributed to the pile’s rear, the spandrel, and the sidewall. The settlement of the surrounding rock and the underground water pressure are effectively controlled, aligning closely with field monitoring results. The pile diameter, length, occlusion thickness, and overlap length significantly affect pile stress, in descending order of impact: occlusion thickness > pile length > overlap length > pile diameter. It is recommended that for pre-reinforcement of Tertiary semi-diagenetic water-rich sandstone tunnels with a surrounding rock water pressure below 300kPa, the horizontal rotary jet pile parameters should be as follows: pile diameter of 65~70cm, pile length of 10~13m, occlusal thickness of 25cm, and overlap length of 3~4m.