The invention relates to antibacterial regenerated cellulose fiber and a preparation method thereof. The antibacterial regenerated cellulose fiber is obtained by solidification of cellulose and a soluble guanidine salt polymer solution, and then regeneration and spinning; and the antibacterial regenerated cellulose fiber contains 0.1-5wt% of the water-insoluble guanidine salt polymer. The preparation method comprises the following steps: cellulose and the soluble guanidine salt polymer are dissolved in a solvent to form a spinning solution, through wet method spinning or dry-jet wet spinning, the antibacterial regenerated cellulose fiber is prepared; wherein, a coagulation bath of wet method spinning or dry-jet wet spinning is a mixing solution of long-chain alkyl sodium salt with the alkyl carbon number on a long chain being 12-18, the solvent, and water. The method has the advantages of simple operation and mild preparation method, the prepared antibacterial regenerated cellulose fiber contains the water-insoluble guanidine salt polymer, has the characteristics if excellent washing resistance, long antibacterial effect, no toxicity, and comfortable performance, has important meaning for developing the cellulose fiber which has the advantages of remarkable antibacterial effect, low cost and long effect, environmental pollution.