This invention relates to a butterfly valve, which includes at least a valve body, a valve shaft, a seal member and a valve plate. Said valve body is provided with a first valve shaft hole, and a through hole is provided in the middle position of said valve body. A positioning ring is provided on the upper periphery of said valve shaft, said valve shaft is composed of an upper section, a middle section and a lower section, said upper section and lower section are cylindrical structures, and said middle section is a non-cylindrical structure; the outer diameter of said positioning ring is larger than the outer diameter of said upper section, a sliding washer is mounted on said valve shaft, and said sliding washer is arranged between the groove of said valve body and the positioning ring of said valve shaft; a fastening recess is provided below the lower section of said valve shaft, and a C-shaped buckle is mounted on said fastening recess. said seal member is provided with a second valve shaft hole, the inner side of said second valve shaft hole is provided with at least one annular flange; the inner side of said seal member is provided with a counterbore at the position of said second valve shaft hole, said counterbore is provided with a resistance-reducing element. Said valve plate is provided with a third valve shaft hole, and the shape of said third valve shaft hole corresponds to the shape of said middle section of said valve shaft.