It has two positions of by-pass and close. It is in by-pass position during trip operation, circulates mud fluid in the drill string into the annular space by-passing the idle motor, so that mo mud may spray out onto the platform during the trip operation. When mud flow rate and pressure reach the setting value, the valve stem moves down and closed the valve. Meanwhile, mud stream flows through the motor, and converts the pressure energy into mechanical energy. As mud flow rate is too low, or mud pump stops, and as the created pressure is not enough to overcome spring force and static friction force, the spring presses the stem upward and by-pass is in open position. In general, the cross-over sub is used in deep well and large angle well horizontal well or hollow rotor selected.