Regarding the disturbance effect on the structure of the existing railway tunnel during the underpass construction of a new shield tunnel, based on the underpass project of Mingyue Mountain Tunnel, a numerical model for the underpass of a new double-track highway tunnel beneath the existing tunnel was established, considering the influences of different factors such as construction methods, underpass angles of the tunnel, surrounding rock types, and burial depths. The settlement and deformation laws of the existing tunnel under the control of different key factors were explored, and the control limits for the influence of the adjacent construction of the tunnel under the control of their respective magnitudes were obtained. The results show that the impact of new tunnel excavation on the existing tunnel structure is reduced when the distance between the new and old tunnels is greater, the properties of the surrounding rock are better, the underpass angle is larger, and the tunnel depth is shallower. The research results can provide references for underpass construction of similar projects and guidance for engineering design.