The groundwater in coastal area is rich and always in contact with the sea, which brings a great risk to the shield launching of seacrossing metro. Taking seacrossing section on Xiamen Metro Line No. 3 for example, an end soil reinforcement method which combines highpressurized jet grouting pile and underground plain concrete diaphragm wall is put forward, in accordance with the technical idea of “sealing and dewatering first and then soil reinforcement”. Meanwhile, corresponding programs of construction dewatering and tunnel entrance sealing are proposed. The construction practice shows that the abovementioned method can effectively block the hydraulic connections between tunnel and sea; thus ensuring the soil reinforcement quality and reducing the risk of shield launching. In addition, the stress state of the underground diaphragm wall under different shield trusting forces is analyzed by numerical simulation software FLAC3D;and then effective control range of shield thrusting force which can guarantee the integrity of underground plain concrete diaphragm wall is obtained, so as to prevent nonplastic fracture of underground plain concrete diaphragm wall.