Abstract: The color of the ammonium sulfate product produced by Huize Smelting Branch of Yunnan Chihong Zn & Ge Co., Ltd. using low-concentration sulfur dioxide flue gas desulfurization is yellow, resulting in unqualified product quality. In this paper, the composition, phase structure and sulfur valence state of the absorption mother liquor and yellow ammonium sulfate product are analyzed by ICP-AES, XRD, XPS, TOC, GC-MS methods. The cause for the yellowing of ammonium sulfate is revealed by comparing the components of unqualified absorption mother liquor with qualified mother liquor, and yellow ammonium sulfate product with qualified ammonium sulfate product and then studying the influence of metal impurities, organic pollutants and sulfur-containing impurities in the absorption mother liquor on the color of ammonium sulfate products. The results show that sulfur pollutants are produced when the thiosulfate in the absorption mother liquor reacts with concentrated sulfuric acid, which is the cause of the yellowing of ammonium sulfate. In addition, feasible solutions, such as adding filtration facilities and using oxidants for oxidation, are proposed.