The invention discloses a desizing method for pure cotton fabrics sized by a starch and PVA (Polyvinyl Acetate) mixing pulp and belongs to the pretreatment technology field of pure cotton fabrics. The pure cotton fabrics are subjected to bio-enzyme desizing treatment dip-padding after being pretreated by using hot water and subjected to stacking treatment, active enzyme remaining in the fabrics is removed by an enzyme deactivation treatment, and then the pure cotton fabrics are washed and dried. According to desizing method provided by the invention, the fiber damage is less, the treated fabrics are high in quality and soft in hand feel; particularly, the chromaticity, pH value and COD value of wastewater treated by the desizing method are greatly lower than that of a conventional process, thus the desizing method is in favor of protection of ecological environment, and the load of sewage treatment is greatly reduced. Because dip-padding-stacking continuous treatment processes are used, the desizing method has characteristics of high production efficiency and low labor intensity, and saves energy and water.