The disclosure discloses a washable flame retardant viscose fabric. The viscose fabric includes a flame retardant viscose fiber; and a method of preparing the viscose fiber includes the following steps: impregnation, squeezing, ageing, yellowing, addition before spinning, spinning, bundling, drafting, cutting off, first washing, desulfurization, second washing, pickling, third washing, application of oil bath, drying, and packaging. Before spinning, an aqueous dispersion of flame retardant, an aqueous dispersion of hyperbranched nanocellulose and a dispersant are uniformly added to a spinning glue using a pre-spinning injection system. During the production of the flame retardant viscose used, the aqueous dispersion of flame retardant, the aqueous dispersion of hyperbranched nanocellulose and the dispersant are introduced into the spinning glue by the pre-spinning injection system before spinning, so that the flame retardant has a high residual rate in the subsequent coagulation bath.