The invention discloses a cotton yarn low-carbon environment-friendly sizing method, which comprises the following steps of (1) using ion liquid for desizing fabrics so that pulp is removed; after the desizing, obtaining mixed liquid containing the slurry and ion liquid; (2) sufficiently contacting the yarns and the mixed liquid for reaction; (3) putting the yarns after the sufficient reaction into a coagulating bath for film forming; (4) recovering ion liquid for cyclic reutilization. The sizing method and the formula provided by the invention are simple; the operation is simple; the pulp composition is cotton fabric desizing mixed liquid; the addition of chemical agents is not needed; the waste water discharge problem is avoided; the problem of environment pollution caused by waste water from water washing after sizing is avoided; the ion liquid in the pulp after the desizing can be used for realizing cyclic utilization.