As more and more construction activities take place adjacent to existing Metro lines, the study on the influence of the foundation pit construction on existing Metro tunnels becomes more and more important. In this paper, the influence of the groundwater seepage on the existing Metro tunnel under different waterinsulating curtain depths is studied by means of FLAC 3D software, with a foundation pit adjacent to No. 1 Line of Suzhou Metro as an example. In the simulation, the displacement field of the Metro tunnel and the stress of the tunnel segments are studied under 2 different depths of the waterinsulating curtains. The study results show that: 1) The increase of the depth of the waterinsulating curtain is helpful for the reduction of the pore water pressure around the tunnel, and in turn helpful for the control of the structural deformation of the tunnel; 2) As the depth of the waterinsulating curtain increases, the stress on the tunnel structure induced by the dewatering in the foundation pit reduces.