Abstract: It poses severe challenges to the normal operation of existing railway lines and the safe construction of the under-constructed projects when the metro shield construction underneath passes through existing railway lines,so the proper construction measures to be adopted are essential. Take a new shield tunnel in Chengdu as the background, a construction method for the shield tunnel underneath existing railway line is discussed, a pipe-roof test is conducted, the countermeasures for risks are proposed, and the monitoring measures and plans for potential risk sources are established. The practical effect shows: the design scheme for the shield passing existing railway line underneath meets the relative specifications and standards requirements of existing railway line; detailed scheme for risk prevention and controlling should be established aiming at different risk sources as for the various projects passing through underneath, and monitoring and measuring should be emphasized to give a scientific feedback to the observational construction.