Abstract: This article makes a theoretical analysis of the air current at the back sheet of a card and propose the concept of gauge ratio,assuming that at the back sheet exists a maximum pressure point whose location is related with gauge ratio,pressure difference,exit gauge and back sheet length.The current status of research on the back sheet of the card is reviewed.It is considered that just additional installation of a web purifier is beneficial to the quality of yarns.And air capacity of 180~240 m3/h is preferred.Additional installation of a stationary flat along facilitate the yarn quality too,larger gauge is preferred.Combination use of web purifier and stationary flat will achieve even better results,and their mounting location should be at the maximum pressure point of the back sheet.