The utility model relates to a waterproof dividing head and a main spindle thereof. The waterproof dividing head comprises a main spindle box and the main spindle rotatably assembled inside the main spindle box. The main spindle comprises a main spindle body and a mounting plate, wherein a circular bulge loop is formed on the surface of the mounting plate; and the axis of the circular bulge loop is superposed with that of the main spindle body. A ring slot is formed on the main spindle box; and the circular bulge loop is correspondingly embedded inside the ring slot. An annular flume is formed on the periphery of the circular bulge loop. A drain hole is formed on the wall of the ring slot, and the opening of the drain hole is downward when the main spindle is horizontal. With the flume on the periphery of the circular bulge loop of the mounting plate of the main spindle and the drain hole on the main spindle box of the waterproof dividing head, cutting fluid and the like flow downwards along the flume under the action of gravity to be drained when entering the clearance between the circular bulge loop and the ring slot of the main spindle box, so that moisture is effectively prevented from entering the main spindle box, and the problem of poor waterproofness between the front end of the main spindle and the main spindle box of the existing dividing head is solved.