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 Lincan-Qingshuihe Expressway in Yunnan Province, considering the geological conditions and the relevant engineering case studies, the horizontal high-pressure rotary jet grouting piles are employed to pre-emptively strengthen the weak surrounding rock. Through the combination of theoretical analysis of foundation beams, numerical simulation and field monitoring methods, the reinforcement mechanism of horizontal high-pressure jet grouting piles in the tertiary semi-diagenetic formations is investigated, and the influences of varying pile parameters on the reinforcement effectiveness of the surrounding rock are examined, offering a suggested range for these parameters to serve as a reference for similar projects. The results indicate that the distribution of shear force and bending moment in both analytical and numerical solutions agrees with that in the unsupported section, where the pile experiences the highest bending moments and shear forces, making it most susceptible to fracture failure. Under the water pressure of the surrounding rock of 300 kPa, the maximum tensile force within the rotary jet grounting pile reaches 598.21 kPa, close to the ultimate tensile strength of the pile. When the water pressure is below 300 kPa, the horizontal rotary jet pile can effectively give full play of the combined effects of beam and arch, significantly enhancing both the arch action and the water-blocking effects. This allows the pressures on the surrounding rock to be redistributed to the pile rear, the spandrel and the sidewall. The settlement of the surrounding rock and the underground water pressure are effectively controlled, which are consistent with the field monitoring results. The pile diameter, length, occlusion thickness and overlap length significantly affect the pile stress, in the descending order of impact: occlusion thickness, pile length, overlap length and pile diameter. It is recommended that for the pre-reinforcement of tunnels in the tertiary semi-diagenetic water-rich sandstone with the water pressure of the surrounding rock below 300 kPa, the parameters of the horizontal rotary jet grouting pile should be as follows: pile diameter of 65~70 cm, pile length of 10~13m, occlusal thickness of 25 cm, and overlap length of 3~4 m.