With the rapid development of urban rail transit, it is becoming more and more common for new subway tunnels to pass through or close to existing building structures, and how to minimize the impact of new tunnels on existing buildings and ensure their structural safety and normal operation has become an important research content in the process of subway construction. The construction of a newly-built tunnel in Zhengzhou is taken as an example, the finite element software Midas GTS is used to establish a numerical calculation model, the deformation influence law of the newly-built tunnel construction on the existing operating subway tunnel is analyzed, and the actual engineering monitoring data is compared. The results show that: 1) After the excavation of the open-cut foundation pit is completed, the maximum vertical deformation and horizontal deformation of the existing subway tunnel are 0.57 mm and 1.21 mm, respectively; 2) After the completion of the underground excavation section, the maximum vertical deformation and horizontal deformation of the existing subway tunnel are 0.27 mm and 0.39 mm, respectively; 3) The actual monitoring data is slightly less than the numerical calculation results, so the results can provide some reference for similar projects.