In order to effectively solve the seepage of diaphragm wall in deep foundation pit engineering, the sonar seepage detection is applied in Jinhu Square Station of Nanning Metro Line 3. Firstly, sonar seepage detection is carried out on 65 waterproof curtains, and the data of seepage velocity, flow direction, seepage quantity and permeability coefficient are obtained. The three-dimensional visualized flow field is inverted and the weak parts of diaphragm wall are accurately located. Then, the grouting anti-seepage treatment is carried out accordingly. Finally, the grouting effect is checked by re-testing, and the seepage quantity of foundation pit is effectively reduced. It provides a complete and feasible solution for the detection and early warning of seepage velocity, seepage risk control management and emergency treatment of seepage accident of diaphragm wall in deep foundation pit engineering.