The invention relates to a grouting device for a joint of an underground diaphragm wall and a construction method. The grouting device is composed of two floral tubes, dense screens and a plurality of steel tubes; the two floral tubes are transversely communicated through the plurality of steel tubes; after being respectively wrapped with the dense screens, the two floral tubes are welded to the joint of two sides of a reinforcement cage of the diaphragm wall; the two floral tubes of the grouting device can be directly utilized for double-liquid grouting and leaking stoppage in case of leaking in the joint of the underground diaphragm wall later; and the two floral tubes of the grouting device can be simply blocked with cement paste in case of no leakage. The grouting device for the joint of the underground diaphragm wall and the construction method have the advantages that the treatment is of high efficiency and at a small cost, and moreover, a wall body is blocked from inside, the effect is obvious, and thus the problem of leaking in the joint of the underground diaphragm wall can be effectively solved.