The invention relates to a twin-screw plastic extruder, which comprises a device for sealing screw tail. The screw tail is sleeved with a sealed room and a sealed ring which are connected by a bolt; the bolt is sleeved with a pressure spring and the sealed room is filled with sealed gas. From another angle, the twin-screw plastic extruder changes mechanism of the traditional device for sealing the screw tail and arranges the pressure spring to keep a compression balance as well as arranges a polytetrafluoroethylene (PTFE) sealed ring to be a friction pair so that good sealing effect and wear resistance are realized. Nitrogen is added in the sealed room and as sealed materials as well as lubricant and coolant of a rotation shaft, the nitrogen blows back leaked materials to an extruder cylinder and seals sealed surfaces easy to leak. The twin-screw plastic extruder completely solves the problems of leakage and corrosion of rotating sealed surfaces and witnesses a great leap of traditional plastic processing technology. Besides, the twin-screw plastic extruder has the advantages of remarkable application effect, leakage and corrosion resistance and prolonging of service life.