Abstract: In order to investigate the mechanism of wool in H_2O_2/NH_2CSNH_2 redox system low temperature dyeing,the advanced analysis such as electron paramagnetic resonance(ESR),gas-chromatography and ultraviolet spectroscopy were used in detection of free radical and the products from the redox system.The results revealed that free radicals from wool fiber and the weak acid dye red B were developed under the H_2O_2/NH_2CSNH_2 redox system in the dyeing solution and this made the molecules on the surface of fiber and the dyes highly reactive.At the same time,the H_2O_2/NH_2CSNH_2 redox system released acid to keep the pH constantly which caused swelling of wool fibers.So that,better dyeing property of wool with weak acid dyes at low temperature was achieved.