Abstract：In the soft region，the optimisation design of diaphragm wall is very important to decrease total investment of whole project. From the survey results of some metro excavations in Shanghai，the designs of diaphragm wall are conservative：both the stress of reinforcing steel and the moment of wall are less than those of the design value. These conservative designs must waste a large amount of fund and urge engineers find ways to solve this contradiction. Through the theory of diaphragm wall design analysed，the allowable width of structure crack wlim= 0.2 mm is confirmed to be the most important factor which caused the negative design. According to the national codes，the ratio of Ms and Mu should be between 0.73 and 0.89. When the criterion of wlim= 0.2 mm was adopted in the design of metro excavation，the ratio value of Ms and Mu of the diaphragm wall was normally less than 0.6. Compared with the wlim= 0.2 mm，this paper points out when the allowable width of structure crack broaden to wlim= 0.3 mm，the design will be more reasonable. By calculating and comparing，a conclusion can be drawn that when the allowable width of structure crack broaden to wlim= 0.3 mm，not only can the complex wall which is composed by diaphragm wall and lining can meet the reduced demand of structure crack under the natural conditions，but also the safety of the excavation can be guaranteed and project investment will be reduced.
