Abstract: From the survey results of some excavations, it is found that the designs of diaphragm wall are conservative:both the stress of reinforcing steel and the moment of wall are less than the design value.Through analyzing the theory of diaphragm wall design, it is found that the conservative design is caused by the allowable width of structure crack w_(lim)=0.2mm.It is analyzed if the allowable width of structure crack is broadened to w_(lim)=0.3mm, the ratio of reinforcement will be decreased, at the same time, the safety of the excavation is guaranteed and project investment will also be reduced.