A new strategy for synthesizing flame retardant viscose fiber (FR-VF) has been achieved by graft copolymerization of acrylonitrile (AN) onto viscose fiber and the subsequent chemical modification with hydrazine hydrate and zinc acetate. The structure of FR-VF was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrometry (FTIR). The surface morphology of FR-VF was analyzed using scanning electron microscope (SEM). The results show that the graft copolymerization of AN onto viscose fiber and the reaction of VF-g-PAN with hydrazine hydrate and zinc acetate have been successfully carried out. The thermal properties tested by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques show that the thermal degradation rate of FR-VF is slower than that of untreated FR-VF after 300°C. As a result, the char residue increases from 5.2% to 50.4% at 800°C. Furthermore, the burned FR-VF keeps original form as evidenced by SEM analysis, indicating that the obtained FR-VF possesses excellent thermal stability and char forming capability.