In order to analyze the influence of the newly-built underpass on the stability of the bridge pier foundation of the existing expressway, taking an underground fuel coal transportation tunnel project as an example, the stability of the foundation is evaluated by the mechanical analytical method, Midas GTS NX finite element modeling software is used to check the calculation. The results show that: 1) according to the mechanical analysis method, the foundation of the existing expressway bridge pier is stable; 2) the deformation of the bridge pier is 0.171 mm, and the comprehensive displacement of the bridge pavement is 0.160 mm, which is less than the limit value of 5 mm required by the design and code; 3) compared with the numerical simulation results and the mechanical analysis results, the influence range conforms to the influence range of 45° + φ/2 defined by the code on the ground surface, but the influence range of the excavation in the range from the excavation tunnel to the ground exceeds the influence range of the fracture angle. The results of this paper can provide reference for similar projects.