Abstract: To quantitatively evaluate the structure safety status and service performance of existing metro shield tunnels after being disturbed by crossing construction, a 3D numerical model of newly-built shield tunnel crossing existing shield tunnels is established by MIDAS software. By adjusting the vertical net distance among tunnels, a quantitative study is conducted on the vertical displacement response of existing tunnels caused by shield tunnel crossing construction under conditions of soft soil strata which is mainly composed of water-rich sand and soft-soil formation in Nanjing region, and based on the mechanical attenuation characteristics of the tunnel, the safety level of the existing tunnel is analyzed. The results show that: the curves of soil mass settlement along the existing tunnel longitudinal direction caused by shield tunnel under-crossing and over--spanning construction are ′W′ type and ′M′ type respectively, and under the same stratigraphical condition, the absolute value of deformation of the existing tunnel caused by over-spanning construction is smaller than that caused by under-crossing construction; the half-groove range of the existing tunnel settlement caused by the water-rich and soft stratum in Nanjing is approximately 3.5 times and 5.0 times the outer diameter of the tunnel respectively. Based on the characteristics of existing tunnel mechanical performance attenuation, the shield tunnel crossing engineering is divided into four categories using the tunnel longitudinal differential deformation as an indicator: weak impact, general impact, significant impact, and intense impact. Based on numerical simulation and historical deformation data, the vertical differential deformation of existing shield tunnels after the completion of three typical shield tunnel crossing projects in Nanjing region is predicted, according to which the corresponding impact level and vertical deformation allowance at the same level are calculated.