The invention discloses a method for dyeing rayon fiber by using reduced dye, and belongs to the technical field of fiber dyeing. The method comprises the following steps: A, pretreating: conveying rayon yarns into a bathtub, adding water, the dye and peregal o, keeping for 9min, then adding caustic soda, then adding bone glue, triethanolamine and water, keeping for 9min, and adding sodium hydrosulfite; B, dyeing: raising the temperature of the bathtub to 60 DEG C, preserving heat for 45 minutes, then adding 5% of the total amount of the sodium hydrosulfite in the step A, and continuing keeping for 15 minutes; C, washing by using water: washing the dyed fiber by using water for 9min each time; D, oxidizing: adding glacial acetic acid and hydrogen peroxide into the bathtub, keeping for 27min, and washing by using water; E, soaping; F, posttreating. By the method, a series of problems which appear in the dyeing process using the vat dye in the prior art are solved; the method is designedfor rayon fiber dyeing, and has the advantages of uniform dyeing and no white core.