A shieldbored Metro tunnel in Shanghai crosses underneath the sensitive section of ShanghaiNanjing railway, therefore the railway authorities have strict requirements on the settlement of the railway tracks. Analysis is made on the risks of the shield tunneling across the existing railway by means of FEM and the following technical countermeasures are proposed: firstly, preconsolidation is made for the ground where the shield passes so as to improve the ground's stability; secondly, strengthened segments with preserved grouting holes are adopted in the tunnel design; thirdly, proper resources are allocated, the construction parameters are optimized and the construction monitoring and measurement are enhanced; finally, the shield tunneling is executed strictly following the predetermined plan. In the end, the shield crosses underneath the existing railway successfully. The paper can provide reference for similar works in the future.