The object of this work is to develop a flame-retardant viscose fiber containing phosphazene derivative. Hexaphenoxycyclotriphosphazene (HPTP) was synthesized and applied to viscose fiber through wet spinning method, with alkyl polysaccharide glycoside as dispersant. Properties of the fiber were tested and discussed. All flame-retardant samples survived 3 ignitions, according to 45 degree slope burning method. Limiting oxygen index value of the flame-retardant fiber containing 16% flame retardant was 28.6 %. The number decreased to 27.5 % after 30 washing cycles. After burning, inflated carbonized coat was found on the fiber surface in scanning electron microscopy image. Thermal gravimetric analysis and differential scanning calorimetry results indicated that the primary decomposition of fiber was moved up by about $20^{circ}C$, while secondary decomposition was delayed by around $46^{circ}C$. Damage on mechanical properties of fibers was insignificant. The introduction of HPTP was compatible and the flame retardancy of viscose fiber was greatly improved with limited negative impact.