Abstract: Adjacent construction of new tunnels can cause disturbance to surrounding soil, resulting in stratum displacement and stress redistribution, posing safety risks to existing tunnels. To effectively control the impact of new tunnel adjacent construction on existing tunnel, it is necessary to study the impact of this construction. Method: Taking HaiTai (HaimenTaicang) Yangtze River Tunnel project as example, the impact of parallel adjacent construction of a subsequent railway tunnel on the existing highway tunnel is analyzed. A threedimensional tunnel simulation model is established to analyze the mechanical response of existing highway tunnel structure. The structural deformation and stress changes of existing highway tunnel caused by parallel adjacent construction under different construction clearances are investigated. Result & Conclusion: (1) The subsequent railway tunnel construction exerts lateral squeezing action on existing highway tunnel. The highway tunnel experiences more significant horizontal deformation than vertical one under the impact of railway tunnel excavation. After railway tunnel excavation, the arch crown settlement, vertical and horizontal convergence deformations of highway tunnel all decrease, and the overall tunnel deviates from the rigid displacement of the subsequent tunnel. The stress in segment, reinforcement, and bolts in highway tunnel also decrease after railway tunnel excavation. (2) Under different construction clearances, the impact of railway tunnel construction on the deformation and stress performance of existing highway tunnel shows a consistent pattern. With the increase of construction clearance, the impact of railway tunnel excavation on highway tunnel gradually decreases, and the highway tunnel deformation and stress change amplitude are generally in decreasing trend.