A waterproof structure for a panel joint of an underground diaphragm wall includes a first waterstop and a first cover plate. The panel joint is provided with a wall connection plate. The underground diaphragm wall has an excavation surface. In a direction perpendicular to the excavation surface, the wall connection plate is arranged below the excavation surface of the underground diaphragm wall. A first accommodating groove is defined between the wall connection plate and the panel joint and adjacent to the excavation surface. The first waterstop is arranged in the first accommodating groove, and both wings of the first waterstop are respectively and adhesively bonded to the two sides of the panel joint. The first cover plate corresponds to the first accommodating groove and is arranged at the excavation surface. The first cover plate is detachably connected to the underground diaphragm wall.