The construction of a shallow-buried large-span and small-clear distance tunnel under a freeway continuous beam bridge in soft soil can cause soil lateral movement, threaten the stability of the bridge pile foundation, and lead to cracking of the continuous beam bridge structure, posing a high risk. In this paper, based on the analysis of the difficulty of a tunnel project, the stress deformation law of continuous beam bridge caused by tunnel excavation is analyzed using numerical methods, and the composite anchor-pile reinforcement technology is proposed. The influence of grouting range and pile length on the stability of bridge structure is studied by numerical modeling and field monitoring experiment. The results show that: (1) The composite anchor pile is reinforced in the core through reinforcement and high-pressure grouting mode, and at the same time as the core forms a small-diameter (15 cm) reinforced concrete pile, the high-pressure grouting strengthens the soil around the pile, and jointly forms a large-diameter cement-soil pile (up to 200 cm). The protected bridge piles are surrounded by a row of piles to achieve excellent protection effect; (2) The composite anchor pile utilizes its own stiffness to isolate the stress release caused by the excavation of the long-span soft soil tunnel, thereby resisting the deformation transmission of the surrounding rock and exhibiting a good stress-strain isolation effect. At the same time, the cement-soil composite pile formed by high-pressure grouting also strengthens the surrounding rock ofthe tunnel, thereby narrowing the plastic failure area of the tunnel and protecting the bridge pile. (3) The grouting range and length of the composite anchor pile have a significant impact on overcoming the deformation of the bridge pier, and when the grouting range reaches 2m, the horizontal displacement of the bridge pile foundation is reduced by 95% after the tunnel excavation is completed. When the composite anchor pile is elongated below the tunnel, the displacement is reduced by 85%, so that the isolation protection can be maximized.