Abstract: In response to the influence of core soil width on tunnel stability during the construction of double-sided wall guide method, this paper takes the Chongqing Rail Transit Line 15 Phase II large section connecting line tunnel project as the background, and studies the effects of different core soil widths on surface settlement, surrounding rock stress, and lining stress through numerical simulation methods. And Abaqus finite element software was used to simulate and analyze four different core soil widths (1.5 m, 2 m, 2.5 m, 3 m). The results of settlement analysis and lining stress analysis indicate that there is an optimal value for the width of the core soil. In this project, the optimal value is 2 meters (corresponding to Scheme 2). When the width of core soil is less than 2 m, the surface settlement and lining stress increase significantly, and the deformation will be more concentrated, which is easy to induce the hidden danger of tunnel structure. When the width of the core soil is greater than 2m, it not only increases the difficulty and risk of excavation, but also the effect of lining stress control is not obvious enough.Therefore, a reasonable reserve of 2 meters of core soil can achieve a balance between deformation control and construction safety. This study can provide useful reference and guidance for the optimization design of core soil in similar large section tunnel projects.