SZ series gearbox for conical twin-screw extruder is a special driving unit matched with conical twin-screw rod extruder. It consists of two parts, namely, the reduction box and the distribution box. After reducing speed and increasing torque of the motor, it outputs the motive power to the distribution box, then drives the double output shafts (the included angle is the same with that of twin-screw rods) via a pair of small conical spiral gears with driving ration of 1:1,thus to drive screw rods to rotate outward with different direction.