The diaphragm wall as a permanent structure has good economic value and more and more engineering applications. However, as the main structure, the bending strength at the joint is difficult to control, which becomes the weak point of wall cracking. Based on the bond-slip constitutive theory of reinforced concrete and the idea of ultimate strength method, this paper establishes the calculation method of upper and lower limit values of flexural capacity of lap joint. Combining with the actual design project of the wall slab of Baishamen Sewage Treatment Plant in Haikou City, the influence law of different overlapping length and spacing of overlapping steel bars on the design value of bending resistance is analyzed, which provides a theoretical basis for optimizing the length and spacing of overlapping steel bars, and further promotes the underground diaphragm wall as a permanent main joint in China. The construction and development of institutions are of great practical significance.