A unidirectionally bypassed bypass valve of a screw drill comprises a bypass valve and a unidirectional valve. The bypass valve mainly comprises a valve body, a valve element, a large spring, a valve orifice, a clamp spring and a retainer ring. The unidirectional valve comprises a body, a valve stem, a small spring and a nut. The unidirectional valve is in matched connection with the bypass valve. The outer tube wall of the body of the unidirectional valve is provided with a connection external thread. The inner wall of the valve port of the bypass valve is provided with a connection internal thread. The unidirectional valve is mounted on the valve port of the bypass valve through the connection external thread and the connection internal thread such that the unidirectional valve is mounted inside the valve port of the bypass valve of the screw drill; an outer port of the unidirectional valve is in direct communication with an outer port of the valve port of the bypass valve, and an inner port of the unidirectional valve is in direct communication with an inner port of the valve port of the bypass valve; in such a way, the valve port of the unidirectionally bypassed bypass valve of the screw drill gives unidirectional bypass, thus overcoming the defect of bidirectional direct pass of the valve port of the bypass valve, and also is simple, stable and reasonable in structure.