The invention relates to a method for preventing a permafrost layer from feeding water to inflow in an excavation pit by using a waterproof film. The method is characterized in that a slot is excavated around a planned pit; the depth of the slot is a depth being 0-100 cm below underground permafrost table; a layer of bentonite or clay with a thickness of 1-30 mm is paved at the bottom of the pit slot; the waterproof film is arranged in the excavated pit slot, and is fully arranged at the bottom of the excavated pit slot; the waterproof film reaches a ground surface along the pit wall, so as to form an L shape and refill and bump with filled soil; and the vertical part of the L-shaped waterproof film along the pit wall is 0.5-5 m far away from the edge which is close to the pit to be excavated. With the adoption of the method provided by the invention, the influence of underground water on an engineering construction process and a frozen earth foundation can be effectively avoided; the water can be effectively stopped by sufficiently considering characteristics of a frozen earth structure, underground water soil properties and characteristics of a hydrological structure, and arranging the waterproof thin film with a special structure around the pit to be excavated. Meanwhile, the water inside the excavated pit can be continuously regulated after engineering construction, so that the purpose of maintaining the stability of the frozen soil foundation for a long period can be realized.