With the development of urban underground space, the spatial interaction between various new pipeline foundation pits and existing underground projects has become the normal. This paper takes a new drainage pipeline trench foundation pit passing through an existing urban tunnel as an example. With the excavation of the trench soil in the upper part of the tunnel, the buried depth of the tunnel is reduced, the earth pressure is relieved, the vertical load is reduced, and the additional deformation and internal force of the tunnel will adversely affect the tunnel structure. This paper puts forward protection measures for decreasing floating and excavation deformation, and analyzes the soil and structural deformation through finite element software to ensure that the impact is controlled within the allowable range, so as to provide reference for similar engineering design, analysis and construction.