According to the settlement detected in the long-term operation of a immersed tunnel, the mechanism of settlement problems that have been neglected by engineers and technicians is analyzed. It is pointed out that the siltation of river channels, reciprocating loads and overloads of tidal tides can cause settlement of immersed tunnels, and the settlement control measures of regular dredging, pipe bottom grouting and pile foundation reinforcement and the calculation method of the reinforced pipe body are proposed. For an actual project, the bearing capacity of the foundation in the reinforcement calculation was analyzed, and the pile foundation reinforcement scheme was proposed, and the internal force and reinforcement of the pipe body itself were analyzed through the structural calculation of the reinforced pipe body.