In view of the quality problems of diaphragm wall,we carry out a statistical analysis on the exploration data,and make clear that the exposed reinforcement and cavity of diaphragm wall are concentrated at the joints above -2.50 m and at the joint of wall sections.The causes of concentrated distribution of defects are analyzed from the geologic conditions,foundation treatment,mud,concrete,grooving,reinforcement cage production and installation,joint pipe placement,etc. Quality improvement measures including detecting mud index and sediment thickness during concrete pouring,increasing the position and perpendicularity detection of butt joint pipe,increasing the perpendicularity requirement of forming groove,improving the protection ability of base plate,improving the stability of the upper soft soil layer and improving the joint form of wall section are put forward.