The deformation of cofferdam structure in uppersoft and lowerhard formation in sea area is a serious problem and should be effectively controlled. The horizontal displacement of retaining structure, support axial force of concrete and surface settelement during foundation pit construction of Su′ai Tunnel in Shantou are monitored, and the relationship between the deformation mechanism of retaining system and foundation pit excavation process is analyzed. Main studies and conclusions are as follows: (1) The occurrence location of the maximum horizontal displacement of the retaining structure is gradually lowered with the increase of the excavation depth of foundation pit; the maximum horizontal displacement of the retaining structure and the support axial force are located in the lower part of foundation pit, which are larger at the west side than east side. (2) The unreinforced section around the foundation pit settles continuously, and the settlement of reinforced section is small. (3) The establishment of comprehensive monitoring and early warning mechanism can guarantee the construction safety of the foundation pit; and some optimization advices for foundation pit support system of similar projects in the future are proposed.