The top pushing underpass of an underpass bridge is an efficient and economical construction plan to solve the problem of route intersection. Clarifying the stress mechanism of the underpass bridge during construction can help optimize the design. Therefore, it is necessary to conduct simulation research on the underpass process of the underpass bridge. A highway underpass bridge in Hunan Province was taken as an example, and based on the finite difference method, numerical simulation analysis is carried out on the underpass top pushing construction process. The law of stress field and displacement field changes during the dynamic construction process of the underpass bridge shield tunneling method is studied. The research results indicate that as the top push progresses, the stress in the soil layer behind the palm face changes significantly, with an impact range of about 6 meters behind the palm face. At the same time, the friction coefficient is closely related to the geological conditions, especially the soil conditions. During the top pushing construction, the friction generated at the bottom of the box culvert accounts for more than 70% of the total friction. The calculation results clarify the stress characteristics and mechanism of the underpass bridge during the top pushing process, which can provide reasonable reference and guidance for similar underpass bridge construction.