The reinforcement area of the shield launching section of the Shantou Bay Tunnel is loosened because of dense boreholes, and the backfill area is blasted by the boreholes. The surface settlement, ground soil pressure, and deep soil displacement around the tunnel in these two areas are monitored. Moreover, the deformation law of boulder formation induced by the slurry shield tunneling with superlarge diameter in the sea area cofferdam is investigated based on shield tunneling parameters such as the pressure of the slurry chamber top, total extrusion pressure, tunneling speed, and cutterhead rotation speed. The following conclusions are drawn. (1) The variations in the tunneling parameters in the reinforcement area are more pronounced than those in the backfill area. (2) The extrusion effect of the ground in the reinforcement area induced by shield tunneling is small, and the surface settlement and deep soil displacement are both within 6 mm. Conversely, the extrusion effect in the backfill area is considerably larger, with surface settlement and deep soil displacement reaching 27 and 17.6 mm, respectively. (3) The deep soil displacement exhibits the phenomenon of first receding from the tunnel, approaching the tunnel, and finally stabilizing. (4) The soil pressure varies during shield tunneling and backfill grouting. The reinforcement area shows a greater soil pressure than the backfill area. (5) The reinforcement and backfill areas are located at the lateral lower parts of the tunnel face. The pressure at the tunnel top is low.