The invention discloses a kit and a method for measuring activity of GCS (glycogen synthetase). The kit comprises reagents as follows: an extraction solution comprising a Tris-HCl buffer solution, a reagent I prepared by dissolving Tris, magnesium chloride, potassium chloride, potassium hydroxide and hydrochloric acid in distilled water, a reagent II prepared by dissolving Tris, magnesium chloride, EDTA (ethylene diamine tetraacetic acid), mercaptoethanol and hydrochloric acid in distilled water, a reagent III comprising lactic dehydrogenase, a reagent IV comprising phosphoenolpyruvic acid, NADH (nicotinamide adenine dinucleotide) and pyruvate kinase as well as a reagent V comprising glycogen, UDPG (uridine diphosphate glucose) and glucose-6-phosphate. According to the method, under the combined action of pyruvate dehydrogenase and the lactic dehydrogenase, the oxidation rate, measured at 340 nm by a spectrophotometer, of NADH reflects the production rate of a reaction product UTP (uridine triphosphate) of GCS, the activity of GCS is further reflected, and the method is high in sensitivity. Currently, no effective kit for measuring the activity of the GCS under the combined action of enzymes on the basis of the spectrophotometry is commercially available.