The invention discloses an improved structure of a screw barrel vacuum chamber of a plastic extruder. The improved structure of the screw barrel vacuum chamber comprises a vacuum withdrawing section and a vacuum chamber, wherein the vacuum withdrawing section is arranged on the rear section of a screw barrel of the plastic extruder, the vacuum chamber is vertically installed on the top of a withdrawing port of the vacuum withdrawing section, the withdrawing port of the vacuum chamber is provided with a vacuumizing pipe with a cooling water passage, a thermocouple installation pipe for storing a thermal couple assembly is arranged in the vacuum chamber, and the thermocouple assembly is connected with a control center of the plastic extruder. The structure is reasonable and reliable, the cost is low, the use is convenient and the effect is obvious. When the vacuum returned material exceeds a set temperature, an alarm can issue an acoustic-optic alarm, a worker is reminded of executing the clearing in time, thus not only can the qualification rate and production efficiency of a finished product be improved, but also the loss of a conventional spare part of the vacuumizing device can be reduced, and the service life of the vacuum device can be prolonged.