The invention provides sodium alga acid flame retardant fibers loaded with metal ions and a preparation method thereof. The sodium alga acid flame retardant fibers are prepared by the following steps: dissolving a water soluble sodium alga acid hybrid material loaded with metal ions in water to prepare a spinning solution, and performing a wet spinning process, wherein the water soluble sodium alga acid hybrid material loaded with metal ions is composed of sodium alga acid, stabilizer molecules and the metal ions with flame retardant effect. The fibers produced by the method disclosed by the invention have the characteristics of high flame retardant property and high breaking strength. The limit oxygen index of the fibers can be 34% and above, the tensile strength of the fibers is higher than 3.5cN/tex, and the linear density is 2.2-3.5dtex.