In this article， the influence of the excavation of the foundation pit of Huanchengnanlu municipal tunnel on the existing underlying shieldbored tunnel on No. 2 line of Xi’an Metro is numerically simulated by means of FLAC3D finite differential software， and the antiuplifting of the existing Metro tunnel is calculated. The calculation and analysis result shows that: 1) The uplifting of the soil mass at the bottom of the foundation pit and the deformation of the existing Metro tunnel can be brought under effective control by using alternative， sectionbysection， layerbylayer and symmetrical excavation of the foundation pit and by consolidating the existing Metro tunnel by means of benchshaped piles. 2) The deformation and uplifting of the existing Metro tunnel is within the limits of related standards， and the safe operation of No. 2 line of Xi’an Metro can be guaranteed. Furthermore， automatic realtime monitoring is made for the existing Metro tunnel during the construction of the municipal tunnel and the monitoring result proves that the design of the construction of the municipal tunnel is reasonable and feasible. 