The invention relates to a joint waterproof pretreatment method in the construction process of an underground diaphragm wall and belongs to the technical field of construction of diaphragm walls. Themethod includes the following steps that S1, a monitoring mechanism is embedded in the joint position of the underground diaphragm wall in the early underground diaphragm wall construction period, andthe monitoring mechanism is used for monitoring the water leakage condition at the joint position of the underground diaphragm wall; S2, a sleeve valve pipe is embedded in the adjacent underground diaphragm wall joint position; S3, grouting and leakage path sealing are conducted in the reserved sleeve valve pipe; S4, when the monitoring mechanism monitors the water leakage condition, a specific water leakage area is determined, and a slotting and joint checking test is conducted in the water leakage area; and S5, filling is conducted in a quality defect area after the slotting and joint checking test is conducted. By means of the method, the monitoring mechanism is arranged in advance for monitoring the water leakage conditions of all the joint positions of the underground diaphragm wall,the sleeve valve pipe which is arranged in advance is used for conducting the grouting and leakage path sealing test, the water leakage area is determined, and meanwhile the water leakage phenomenonhappening to the joint positions in the subsequent excavating process is prevented through the slotting and joint checking test.