Abstract: Objective: Shield tunnel excavation causes the overall structure uneven settlement of existing highspeed railway bridges in the surrounding area, thereby affecting their normal functionality and operational safety of train. In this context, based on an interval tunnel of Nanchang Rail Transit Line 4 phase I project, the impact of shield tunnel underpassing existing highspeed railway bridge and the reinforcement schemes are analyzed.Method: The process of shield tunnel underpassing existing highspeed railway bridges is simulated using Midas GTS software. The development law of displacement and settlement on the top of highspeed railway bridge piers and abutments are analyzed, as well as the deformation before and after reinforcement.Result & Conclusion: Shield tunneling has the greatest impact on the settlement of bridge piers and abutments, with a maximum settlement of approximately 3.40 mm and a maximum longitudinal displacement of about 2.50 mm. The impact on lateral deformation is minimal, with a maximum lateral displacement of about 1.00 mm, mainly occurring in the initial excavation stage. The deformation of highspeed railway piers caused by shield tunneling decreases with the subsequent installation of tunnel segments and grouting. After implementing isolation piles + grouting reinforcement measures, the maximum settlement of bridge pier tops is approximately 1.54 mm, with a maximum settlement difference of 0.34 mm and a maximum longitudinal displacement of 1.41 mm. The maximum settlement of the pier and abutment tops meets the control standard requirement (3.00 mm) for highspeed railway bridge pier settlement and allows for a certain deformation margin, being relatively safe. The reinforcement measures of grouting and isolation piles perform well, being safe and feasible.