The flame retardant fiber is based on viscose fiber, and the flame retardant additives are added to the spinning stock solution, so that the spinning stock solution and flame retardant additives are mixed into one, and the flame retardant additives are evenly distributed inside the fiber. When the fiber is burned in the open fire, the flame retardant is decomposed by heat and releases inflammable gas to cover the fiber surface, prevent the reaction between oxygen and cellulose, and the cellulose is dehydrated and carbonized. Form a flame retardant carbon layer to achieve the purpose of flame retardant.