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The article relies on the Guang-Xing section of the southern section of the third phase of Chengdu Metro Line 1 to cross the Cheng-Hua Railway. The change of the stress with the approaching is analyzed. Study the stress change law of the intercalated rock under the change of the distance and angle: when the distance decreases, the maximum principal stress increases more obviously, which shows that the smaller distance, the easier the damage of intercalated rock. The stress state of the rock at different angles is that it is safer to overpassing and undercrossing, the horizontal approach is the most dangerous, and the other angles tend to be between the two. Through three-dimensional numerical simulation, the influence of the excavation process and construction method is analyzed, and it is found that the change of the secondary stress on the sandwiched rock by the mining method is relatively larger than that of the shield method. This makes the surrounding rock disturbance relatively large, resulting in the weakening of the sandwiched rock, which is directly manifested as the increase of the subgrade displacement.

In order to analyse the safety of the two main line tunnels and two ramp tunnels crossing the existing Bajiaogou tunnel in Chongqing University City, the plane and three-dimensional models were established to study the impact on the existing railway tunnels by strength reduction method and structure analysis method. Research results: the influence of different excavation bench lengths (6m and 12m) on the existing tunnel is analysed, and suitable excavation bench length is 6m. According to the deformation rule of the lining rigid body structure, the displacement control value of the C35 lining structure is 5.9mm. Various excavation procedures have different effects on the structure and the overall stability of the surrounding rock, so optimizing the excavation process can reduce the impact of construction. Optimization principles are derived from the structural displacement time history curve of the optimal conditions. The principles: interval excavation of approaching large-section tunnels, avoiding multiple disturbances in a short time, excavating the distant tunnel before the vicinity and excavating the upper tunnel before the below. The conclusions have guiding significance for the optimization of the four large-section tunnels’ approaching construction.