The utility model discloses a free purification unit is retrieved to synthetic low pressure of polyvinyl chloride, the high -pressure pure chlorine ethylene gas that will come from polymerization divides division processing recovery with the chlorine ethylene gas that low pressure inert gas content is high, and with low pressure chlorine ethylene gas through the dewatering, retrieve again after the edulcoration to mix with fresh chlorine ethylene gas and throw the material once more, the qualities of retrieve vinyl chloride are improved, and then pvc resins's product quality has been improved, the production system traveling comfort has been improved simultaneously.