The invention relates to the technical field of buildings, and discloses a watertight test method for a waterstop. The method comprises the following steps: S1, mounting a middle buried type waterstop along a joint of a concrete wall body in the joint, and mounting an external waterstop at a position, which is opposite to the joint, outside the concrete wall body; S2, arranging a first pre-buried water pipe and a second pre-buried water pipe which are communicated with the external waterstop; S3, connecting a water discharging pipe of a watertight test machine with a water flowmeter and then with the first pre-buried water pipe; S4, pressurizing step by step by the watertight test machine till a pressure value meeting the design requirement is reached, and shutting off a stop valve of the first pre-buried water pipe; S5, checking whether water leakage and abnormity exist around the middle buried type waterstop and the external waterstop, and checking the water flow change of the water flowmeter; S6, if the instantaneous flow is zero, and the total flow is not changed, indicating that the waterstops are qualified in mounting, otherwise, indicating that the waterstops are disqualified in mounting. The method is simple in test and precise in result.