When a new tunnel closely passes through an existing shield tunnel, it inevitably causes the deformation and settlement of the existing tunnel and changes the stress state and distribution of the tunnel structure. Based on the project of the shield tunneling undercrossing airport special line of Xiba River-Sanyuan Bridge section of Beijing Line 12, a modified structure model for the existing tunnel was developed. The model takes the top surface of the rail as the position of the neutral axis, which can better consider the impact of the operating tunnel on the track bed, steel rail and other structures. Numerical methods were used to study the influence law of tunnel settlement caused by underpass construction under different longitudinal stiffness of existing tunnels. By comparing with the monitoring data, it is found that the predicted value of the revised model is the most consistent with the monitoring value, which verifies the applicability of the model in this project. Parametric analysis was carried out from the two aspects of tunnel clearance and soil parameters. The research results can provide references for the similar projects.