The invention discloses a method for preparing alginate/cellulose composite fibers, which comprises the following steps: (1) impregnating cellulose pulp with solution of NaOH for alkalization and obtaining alkaline cellulose; (2) adding sodium alginate in an amount which is 1 to 5 weight percent based on the weight of the cellulose into the alkaline cellulose, and mixing uniformly; (3) adding CS2 into the mixture to perform a sulfonation reaction, adding deionized water for dissolving the product of the sulfonation reaction, and obtaining cellulose/alginate xanthogenate viscose; (4) filtering, deforming and aging the cellulose/alginate xanthogenate viscose to obtain viscose spinning solution; and (5) performing wet spinning molding of the viscose spinning solution, solidifying and regenerating the molded product to obtain the alginate/cellulose composite fibers. The alginate/cellulose composite fibers prepared by the method are applicable in the field of health and medicine and can be used for making underwear.