The utility model provides a polycarboxylate water reducing agent's vacuum drying device, it includes drive device, vacuum drying machine and extraction apparatus. Drive device has the drive shaft. Vacuum drying machine has: main shaft, main shaft set up in vacuum drying machine and are connected with the drive shaft, steam inlet sets up and inclines under vacuum drying machine, condensate outlet sets up and inclines under vacuum drying machine, stir the board, set up on the main shaft, first discharge gate sets up and inclines under vacuum drying machine. The extraction apparatus has: charge door and second discharge gate set up downside on the extraction apparatus respectively, the stirring rake sets up within the extraction apparatus, be connected through pipeline between extraction apparatus and the vacuum drying machine. It has following advantage: the device adopts two steps of seasonings, with vacuum dehydration again after the polycarboxylate water reducing agent extraction dehydration, can not only and make it keep the performance the same with stoste at dry polycarboxylate water reducing agent under the lower temperature earlier, and simultaneously, drying efficiency improves, greatly reduces the energy consumption.