A rice field adjustable folding type downflow weir device is provided with an outer plastic frame, an inner plastic frame, an upper plastic border frame, a weir ridge, a metal wire and a bolt. The outer plastic frame is arranged between ridges of a rice field in a straddling mode, the lower end of the outer plastic frame is vertically inserted to the position below a field face, and the face, close to the ridges, of the outer plastic frame is subjected to pouring treatment through concrete and bricks. The inner plastic frame is vertically connected into the outer plastic frame and provided with a guide rail and a positioning hole. The weir ridge comprises a plurality of anti-aging hard plastic plates, every two plastic plates are tightly hinged in series, and the metal wire is embedded to the centers of the plastic plates and vertically extends into the guide rail. The upper plastic border frame is arranged at the upmost end of the weir ridge, and the bolt is arranged on the upper plastic border frame to carry out positioning. The invention further discloses an application method of the device. The rice field adjustable folding type downflow weir device is low in cost and high in durability, the flood resisting character of rice crops is fully utilized, the water storage depth of the rice field is precisely adjusted, the hydrological regulation and storage capacity of the rice field is enhanced, a good rice field nitrogen and phosphorus contaminant controlling and discharging effect is achieved, an effective technical measure is provided for farmland non-point source pollution treatment, and the rice field adjustable folding type downflow weir device is suitable for being popularized on a large scale.