The invention relates to the technical field of synthetic resin and plastics, in particular to a preparation method of a viscose fiber subjected to blending modification with methoxypolyethylene glycol phosphazene. The preparation method of the viscose fiber subjected to blending modification with methoxypolyethylene glycol phosphazene comprises the following steps: (1) synthesis of methoxypolyethylene glycol phosphazene; (2) purification of methoxypolyethylene glycol phosphazene; (3) preparation of blended flame-retardant viscose fiber. The prepared viscose fiber subjected to blending modification with methoxypolyethylene glycol phosphazene has good flame-retardant effect, and the limit oxygen index of the flame-retardant modified viscose fiber reaches 33%.