A shieldbored running tunnel of Line S8 of Nanjing Metro crosses underneath existing NiQi railway. It is of great importance to ensure the normal operation of the existing railway during the shield boring. In the paper, the deformation of the ground caused by shield boring is analyzed by means of FLAC 3D numerical model, and comparison and contrast is made between the measured deformation and the calculated deformation. Conclusions drawn are as follows: 1) In addition to the grouting reinforcement made to the ground, simultaneous grouting and secondary grouting shall also be strengthened during shield boring, the critical control zones shall be established and specific shield boring parameters shall be determined for each segment ring; 2) Due to the ground reinforcement measures taken, the settlement of the existing railway caused by shield tunneling can be reduced to 0.7 mm; 3) The monitoring result shows that the ground surface settlement of the subgrade of the existing railway is more serious than that of the bridge and culvert, but it is still within the limit. 