The invention relates to the technical field of radial and horizontal drilling in oil fields, in particular to a reverse flushing device for a radial and horizontal drilling window cutting filter. The upper end of a valve body of the device is in threaded connection with the lower end of a centering sleeve; a lower joint is designed at the lower end of the valve body; the interior of the valve body is fixedly provided with an inner cylinder; a plurality of overflow holes are evenly distributed in the side wall of the inner cylinder; an annular liquid cavity is formed between the outer wall of the inner cylinder and the inner wall of the valve body; a valve core is arranged in the inner cylinder; the lower end of the valve core is designed to be a cone which can be matched and sealed with the conical surface of the inner wall of the valve body; the valve core is sleeved with a spring; and the upper end of the spring bears against the inner end face of the upper part of the inner cylinder. According to the reverse flushing device disclosed by the invention, the downhole window cutting filter can be cleaned without pulling out a tubular column, operation is simple, normal construction operation is not influenced; and work efficiency is greatly increased, operation time is shortened, and operation cost and labor strength of a worker are reduced.