The invention provides starch textile size, a preparation method thereof and a warp sizing method. Plasma treatment is conducted to the starch textile size to obtain modified starch textile size. By sizing the obtained size on warps, since the plasma treatment is conducted, the molecular weight of the size is reduced and the viscosity is low, the concentration of the size can be improved; and moreover, after the size is undergone through the plasma treatment, hydrophilic groups are added, the size activity and the water solubility are increased, the encapsulating performance of the size is improved, the fiber adhesion performance is good, the yarn breakage rate is decreased, the sizing effect is better and the mechanical performance of the obtained sized yarns are improved. The warps obtained after sizing by using the size provided by the invention can facilitate the desizing before printing and dyeing in the next process, the size can be more easily removed from fabrics and the desizing rate of the fabrics is improved. According to experiments, by using the size provided by the invention, the desizing rate of the fabrics can be improved to 92.5 percent, and compared with the traditional technology, the desizing rate is improved by more than 20 percent.