The invention discloses a low-pressure detector for waterproofness of a thin waterproof coiled material. The detector comprises a water tank, a pressure gauge, a circular pressure base, an upper pressure flange, an air exhaust valve and a leather hose, wherein the volume of the water tank is higher than three times that of the circular pressure base; an air exhaust pipe pass through the upper part of the wall of the circular pressure base; the outer part of the air exhaust pipe is connected with an air exhaust valve, and the inner part of the air exhaust pipe is extended into a cavity of the circular pressure base; an opening of the air exhaust pipe is positioned at the lower position of the upper edge of the circular pressure base; a plurality of pressure screw rods are arranged on the upper part of the outer wall of the circular pressure base; and a lower rubber sealing washer, a thin waterproof coil material testing sample block, lower laboratory filter paper, a moisture indicatingmixture, upper laboratory filter paper, circular organic glass and an upper rubber sealing washer can be arranged between the circular pressure base and the upper pressure flange, and the circular pressure base and the upper pressure flange are connected with each other in a movable way through a plurality of pairs of pressure nuts and the pressure screw rods on the circular pressure base.