Abstract: To reveal the deformation characteristics of deep foundation pit excavation for open-cut tunnels using cofferdam construction under thick soft soil foundations in sea areas, relying on the west end section of foundation pit excavation with cofferdam in Shenzhen-Zhongshan Link, the on-site measured data were statistically analyzed and a fine numerical model was established using PLAXIS 3D finite element software. The calculation results and measured data were compared and analyzed, and the excavation characteristics of the foundation pit in the soft soil layer in the sea area were obtained. The results show that: (1) During the excavation and supporting process of the foundation pit, the diaphragm wall undergoes "bulging" deformation towards the inside of the pit, with the maximum horizontal displacement of the wall being 57.15 mm, reaching 0.34%H_e (H_e is the excavation depth), located about 13 m below the excavation surface. (2) The surface settlement is relatively stable during the construction period of the tunnel base slab, and shows a subsidence trend in other periods, with the maximum settlement value reaching −33.68mm. (3) In the steel-concrete composite support structure, the maximum axial force of all supports are concentrated on the concrete supports within the depth range of 6.4 m-7.9 m, and the maximum axial force among all supports is 13,634 kN. (4) Under soft soil geological conditions, the numerical calculation results of the lateral displacement of the diaphragm wall are close to the measured data, and the deformation law is consistent with the actual project.