Construction process of shield undercrossing the existing metro tunnel No.1 is monitored. Based on monitored data, the heave/subsidence, horizontal and convergence displacement of the existing tunnel structure are analyzed with the change of the relative position of the shield machine to the existing tunnel. Shield driving parameters have been recorded during the tunneling process. The parameters of shield machine are optimized further through the feedback of line 1 monitoring results, and the existing tunnel deformation is controlled within the permitted value according to the reasonable set of shield parameters. Based on the deformation of built tunnel, it is found that the displacement of existing tunnel is negligible when the cross tunneling is more than 20 meters away. The shield process had a major impact on the left side of the existing tunnel. The results indicate that the displacement process of the interaction of existing tunnel and new tunnel can be divided into five phases.