The invention belongs to a device and method in the technical field of building engineering, and provides a joint for an underground diaphragm wall and a construction method of the underground diaphragm wall. The joint is formed by combining a U-shaped steel plate to an I-shaped steel plate; and one end of the I-shaped steel plate is connected with the bottom of the U-shaped steel plate along thedirection of a symmetry axis. The construction method of the underground diaphragm wall comprises the following steps: arranging the joint at the position of a seam between two diaphragm walls; transversely placing the joint in the diaphragm wall; welding an I-shaped end of the joint to one side of a binding reinforcement cage and enabling the other side of the binding reinforcement cage to be convex; inlaying a convex binding end of a rear reinforcement cage into a U-shaped groove of the front diaphragm wall; and pouring a rear concrete diaphragm wall. The joint integrates characteristics ofimpermeability and watertightness of an I-shaped steel plate joint and an H-shaped steel joint, one end of the joint is welded to the reinforcement cage, integrity is maintained, the other end of thejoint enables the reinforcement cage to be inlaid in the U-shaped groove, contact area and folding points are increased, and the waterproof effect is enhanced; and the construction method of the diaphragm wall is simple and easy to operate, good in waterproof effect and relative low in cost, and has quite high application and promotional value.