Abstract: Objective: In the case of constructing a new subway shield tunnel crossing an existing diseased railway bridge in a rich water sand layer, due to the possibilities of increasing construction risks. A risk analysis of the construction needs to be carried out, and specific control measures need to be formulated to address the risks. Method: The project of crossing Beijing-Kowloon Railway by Nanchang rail transit Line 4 from Fenglu Station to Qili Station is overviewed，the construction risks of the project are identified and analyzed. Based on the FLAC3D software, a numerical model is established with matching material parameters to simulate the shield tunnel excavation process. Three construction conditions are set, and numerical calculations are performed on ground subsidence and existing railway subgrade settlement under different conditions. It is determined that the project should adopt Construction Condition III (layer grouting reinforcement + framed culvert reinforcement) as the construction control measure. Regarding this measure，a monitoring plan for actual constructions is formulated, and the measured data are analyzed. Result &Conclusion: Reasonable parameters for shield machine tunneling can effectively reduce ground settlement and railway subgrade settlement. The above control measures can effectively address issues such as high sensitivity of railway diseased bridge and poor overall stability, thus reducing the risks of shield tunneling construction.