Tertiary amine groups can exhibit and lose cationic properties at different pH levels, and this group can be introduced to starch chains to satisfy the demand for sizing and desizing. However, the effect of pH on tertiary amine groups in toughening starch film and improving adhesion is still unclear. Therefore, 2-dimethylaminoethyl starch (DMAES) was prepared via tertiary aminated starch with 2-dimethylaminoethyl chloride hydrochloride. The influences of pH on the mechanical properties of DMAES films and the adhesion to cotton fiber were investigated. In addition, the desizability and biodegradability of DMAES were measured. It was found that introducing the group of 2-dimethylaminoethyl could toughen the starch film and increase the adhesion to cotton fiber. DMAES has a higher zeta potential and the adhesion to cotton fibers in acidic conditions than in alkaline conditions. In addition, it is easy to remove the DMAES from the yarns and obtain higher desizing efficiency with alkali desizing. Tertiary amination of starch with the 2-dimethylaminoethyl group was a good way for starch to improve adhesion and obtain a higher desizing efficiency.