Abstract: Because the size pick-up for the pre-wet sizing process cannot be obtained according to the relationship between the extrusion moisture regain of the sized yarn and the size pick-up,the method for predicting the size pick-up for the pre-wetting process is studied by the relationship between the extrusion moisture regain of the sized yarn and the size pick-up.A pilot sizing system with pre-wetting device is built,by which the experiment of the traditional sizing and pre-wet sizing for 100% cotton yarn is carried out respectively,with the size concentration changing between 8%～11%.Then the size pick-up of the sized yarn obtained in the above experiment is measured.Based on the established relationships between the size pick-up and the extrusion moisture regain of the sized yarn,the size pick-up of sized yarn in pre-wet sizing process is predicted.The predicted results show that the size pick-up of the sized yarn can be exactly predicted according to the extrusion moisture regain of the sized yarn and the decrease coefficient of the size concentration when the size concentration is given.