The invention discloses a method for constructing an underground diaphragm wall joint. The method refers to a slurry tank, a plurality of diaphragm wall channel sections are formed in the slurry tank, seams are formed between the adjacent diaphragm wall channel sections, a positioning piece is arranged at the top of each diaphragm wall channel section, the tops of the positioning pieces are connected with leakage detecting chambers, the bottoms of the leakage detecting chambers communicate with a plurality of ventilation pipes, the positions, corresponding to the ventilation pipes, of the tops of the leakage detecting chambers communicate with second leakage detecting channels, and the tops of the second leakage detecting channels communicate with mounting grooves. The positioning pieces are pre-buried in all the diaphragm wall channel sections correspondingly, the leakage detecting chambers are formed in the tops of the positioning pieces, permeability detection is conducted on the interiors of the leakage detecting chambers, after it is detected that no air leaks, leakage detection is conducted through the second leakage detecting channels, cavities and the ventilation pipes, if it is detected that leakage occurs, concrete is directly introduced for leaking stoppage, after leaking stoppage, construction can be completed only by pulling out the positioning pieces, simpleness and convenience are achieved, and leakage detecting of all the diaphragm wall channel sections in the slurry tank can be completed.