In order to clarify the mechanical response characteristics of cellular diaphragm wall for drydock foundation pit excavation of immersed tunnel, a threedimensional calculation model is established by the finite element software based on a typical engineering example. The mechanical characteristic of cellular diaphragm wall is revealed. The support scheme evaluation and optimization are carried out, and the sensitivity of deformation to different influencing factors are explored. The results show that: (1)Displacement catastrophe occurs at the joint between the two support forms in the foundation pit, so it is necessary to strengthen the joint on the basis of the original support scheme. (2)Positive angle on both sides of the cellular diaphragm wall move toward to the pit, which results in obvious stress concentration at the positive angle of the cover plate and local cracking with a tendency of gradual failure. (3)Add tensile steel bars at the stressconcentrated region so as to restrain the formation and expansion of crack. (4)Deformation of cellular diaphragm wall is obviously affected by the thickness of cover plate, while it is slightly affected by the reinforcement range of the mixing pile. (5)With the reduction of joint stiffness between walls, the overall space stiffness of cellular diaphragm wall decreases, and the displacement increases gradually. When the stiffness decreases to a certain extent, it tends to be stable.