Abstract: Aiming at the deformation control difficulties of shield tunnels undercrossing high-speed railway bridges in complex soft soil conditions, this study takes the project of Hangzhou Gaojiao Road Tunnel underneath Yuhang Extra-large Bridge of Hefei-Hangzhou High-speed Railway as the research object, and carries out research on comprehensive technology and deformation law through the perspectives of route selection, structural optimization, support system design and numerical simulation. Firstly, combined with the constraints of the surrounding sensitive environment, the single-tunnel double-track stacked shield structure scheme is determined through multi-alternative comparison. Secondly, a combined support system of "isolation pile + grouting reinforcement" is established targeting the project characteristics, and the pile type, grouting parameters and layout range are optimized. Furthermore, PLAXIS 3D is used to establish a three-dimensional numerical model to analyze the displacement evolution law of bridge piers and pile caps. The results demonstrate that the adopted route type, tunnel cross-section structure and combined support system are safe and reliable. The research achievements can provide technical references for similar projects of shield tunnels undercrossing high-speed railway bridges in complex soft soil conditions.