The invention discloses a method for ensuring the construction quality of a polyurethane protection system of a water-rich dam. A bag fixing seat is fixed relative to a V-shaped drill bit, the V-shaped drill bit is sequentially coaxially connected with a plurality of drill rods, the V-shaped drill bit and the plurality of drill rods are wrapped by a waterproof bag, after the V-shaped drill bit ispressed into a grouting hole to form a hole mold, the bag fixing seat is separated from the V-shaped drill bit by high-pressure air, meanwhile, an air compressor enables the waterproof bag in a contracted state to be expanded and enter the hole mold, supporting of the bag fixing seat and the waterproof bag for the hole mold is ensured, and therefore, after the V-shaped drill bit is lifted up and taken out from the grouting hole, closing of the formed hole mold caused by the effect of surrounding stress is avoided. In addition, polyurethane is injected into the waterproof bag, polyurethane is foamed in the waterproof bag so as to be separated from water in a dam, therefore, the foamed and cured polyurethane quality is ensured, the quality of the whole polyurethane protecting system of the dam is ensured, and the dam reaches the needed anti-seepage reinforcing effect.