A butterfly valve, comprising: a valve body being cylindrical, formed integrally and having an inlet end, an outlet end, and an axial passage communicating with the inlet end and the outlet end, wherein the valve body is provided with a valve stem mounting hole in a radial direction between the inlet end and the outlet end, the interior of the valve body is provided with a valve seat mounting portion at a position corresponding to the valve stem mounting hole, and the valve body is provided with sealing surfaces (1014a, 1014b); an inner valve seat fixedly mounted to the valve seat mounting portion, wherein the inner valve seat is cylindrical, and a radial through hole corresponding to the valve stem mounting hole is provided within a side wall of the inner valve seat, wherein the inner valve seat has two axial side ends, namely a rear side end and a front side end, wherein both the rear side end and the front side end are provided with a respective flange (1041, 1042) protruding radially outward; a valve core being a plate-shaped member and mounted in the inner valve seat, wherein a radial through hole is provided inside the plate-shaped member; a valve stem passing through the valve stem mounting hole, the through hole of the inner valve seat and the through hole of the valve core successively; wherein the butterfly valve further comprises at least one positioning ring (106,107) mounted at an axial side of the inner valve seat inside the valve body and forming a hermetic connection with the inner valve seat, wherein the inner valve seat is provided with at least one of the at least one positioning ring (106, 107) on both axial side ends, wherein an annular surface (1041a, 1042a) on one axial side of each of the flanges (1041, 1042) forms a sealing contact with the respective sealing surface (1014a, 1014b) of the valve body, characterized in that the other axial side of the flange (1041, 1042) forms a sealing contact with a respective sealing surface (1061, 1071) of the respective at least one positioning ring (106, 107); and the inner valve seat is provided with a sealing rib cooperating with the respective sealing surface (1014a, 1014b) of the valve body. a valve body being cylindrical, formed integrally and having an inlet end, an outlet end, and an axial passage communicating with the inlet end and the outlet end, wherein the valve body is provided with a valve stem mounting hole in a radial direction between the inlet end and the outlet end, the interior of the valve body is provided with a valve seat mounting portion at a position corresponding to the valve stem mounting hole, and the valve body is provided with sealing surfaces (1014a, 1014b); an inner valve seat fixedly mounted to the valve seat mounting portion, wherein the inner valve seat is cylindrical, and a radial through hole corresponding to the valve stem mounting hole is provided within a side wall of the inner valve seat, wherein the inner valve seat has two axial side ends, namely a rear side end and a front side end, wherein both the rear side end and the front side end are provided with a respective flange (1041, 1042) protruding radially outward; a valve core being a plate-shaped member and mounted in the inner valve seat, wherein a radial through hole is provided inside the plate-shaped member; a valve stem passing through the valve stem mounting hole, the through hole of the inner valve seat and the through hole of the valve core successively; wherein the butterfly valve further comprises at least one positioning ring (106,107) mounted at an axial side of the inner valve seat inside the valve body and forming a hermetic connection with the inner valve seat, wherein the inner valve seat is provided with at least one of the at least one positioning ring (106, 107) on both axial side ends, wherein an annular surface (1041a, 1042a) on one axial side of each of the flanges (1041, 1042) forms a sealing contact with the respective sealing surface (1014a, 1014b) of the valve body, characterized in that the other axial side of the flange (1041, 1042) forms a sealing contact with a respective sealing surface (1061, 1071) of the respective at least one positioning ring (106, 107); and the inner valve seat is provided with a sealing rib cooperating with the respective sealing surface (1014a, 1014b) of the valve body.