Proposed in this paper is a UV-based method to rapidly determine the degree of substitution（DS） of carboxymethyl cellulose（CMC） with high substitution.In this method,a curve of the UV absorbance of CMC-Cu（Ⅱ） solution versus the volume of dilute copper sulfate solution is obtained via the reaction CMC and Cu（Ⅱ）,which is divided into three regions,of which regionⅠ defined by Lambert-Beer＇s law corresponds to the stoichiometrical reaction of the carboxyl groups on CMC chain with Cu（Ⅱ） ions.Then,by fitting the curve in region Ⅰ,a straight line is obtained.When the square of correlation coefficient is more than 0.990,the DS of CMC can be calculated according to the volume of dilute copper sulfate solution at the intersection with maximum absorbance.The proposed method,which is of the advantages of rapidity,simplicity and is of small quantity of sample involved,is accurate in determining the DS of CMC with a DS higher than 0.80.As compared with the national standard method,the error of the proposed method ranges from 1.7% to 3.5% only.