The invention discloses a method of preparing an anti-pilling finishing liquid for a worsted tweed. The method comprises the steps of preparing a sweeping agent solution, extracting wool keratin, adding an indifferent oxide steady solution and an ionic liquid, and the like. The method of preparing the anti-pilling finishing liquid for the worsted tweed is implemented by way of combining a phosphoric ionic liquid with a keratin solution, in which the hydrophilic groups of the cationic fraction in the phosphoric ionic liquid cause irreversible changes in the fabric, including dissolving part of scale layers and removing edges and corners from scales to expose more active sites on the surface of the wool; the active groups of the keratin form hydrogen bonds with the active groups of the wool to form a netted film having a certain strength on the fiber surface of the wool, with the netted film functioning in reducing the directional friction effect of the wool and the crimp elasticity of the wool, thus achieving the anti-pilling effect. In addition, the addition of the ionic liquid is conducive to decreasing the usage of the keratin.