The protection effect of the division excavation of foundation pit on adjacent rivercrossing tunnel is studied by taking the foundation pit of Lyugu Phase 1 Project adjacent to doubletrack rivercrossing tunnel on South Tibet Road in Shanghai for example. Firstly, the influence of division excavation and nondivision excavation of foundation pit on displacement of underground diaphragm wall and convergence deformation of the existing rivercrossing tunnel is studied by finite element method. And then, the deformation rules of the existing rivercrossing tunnel and underground diaphragm wall under division excavation of foundation pit are analyzed based on site monitoring data. The study results show that: (1) The maximum horizontal displacement of underground diaphragm wall is reduced by 23.9% and the maximum vertical displacement of the rivercrossing tunnel is reduced by 35.4% by adopting division excavation for foundation pit; and the protection effect of division excavation of foundation pit on tunnel closed to the foundation pit is better than that away from foundation pit. (2) The deformation of underground diaphragm wall increases with the growth of division area of foundation pit. (3) Oblique irregular deformation of rivercrossing tunnel occurs during foundation pit construction, and the deformation of the adjacent rivercrossing tunnel can be predicted based on the maximum horizontal displacement of underground diaphragm wall.