The invention discloses an antibacterial fiber and a preparation method thereof, belonging to the technical field of spinning. According to the technical scheme, the antibacterial fiber structurally comprises a cotton fiber core layer and a nano-silver/cellulose composite surface layer; the preparation method comprises the following steps: (1) soaking the cotton fiber in a nano-silver solution, adjusting the pH to be 7-8, soaking for 5-30 minutes, taking out and drying to obtain the cotton fiber capable of adsorbing the nano-silver; (2) soaking the cotton fiber capable of adsorbing the nano-silver into a cellulose-based xanthate salt solution for 5-60 minutes, taking out and drying to obtain the cotton fiber adhered with the nano-silver; and (3) soaking the cotton fiber adhered with the nano-silver into a sulfuric acid solution for 5-30 minutes, taking out and drying to obtain the antibacterial fiber. The antibacterial fiber has the beneficial effects that (1) the antibacterial fiber is excellent in antibacterial property and good in washing fastness; and (2) the antibacterial fiber is ingenious in design and adopts a simple production method, and has great application potential in the fields of garment and textiles, and industrialized production is easy to realize.