The invention relates to a polysaccharide quantitative detection method and system. In this method, polysaccharide was separated by chromatographic column, hydrolyzed and reacted with fluorescent derivatization reagent, and the generated fluorescent derivative was determined by fluorescence detector to obtain the content of polysaccharide. The polysaccharide detection system of the invention sequentially comprises a high performance liquid chromatography column, a hydrolysis reactor, a fluorescence derivatization reactor, a cooling tube, a fluorescence detector and a workstation or a recording device. The detection system of that invention greatly improve the detection sensitivity by hydrolyzing polysaccharide first, and then react with a fluorescent derivative reagent, and is suitable for quantitative detection of various types of polysaccharide content, and the detection limit can reach tens of ng.