The invention discloses ultra-low loss optical fibers. Alkali metal elements are added during the VAD deposition technological process to reduce the viscosity of a core layer, enable the core layer tobe more matched with an inner cladding layer and an outer cladding layer, and reduce the internal stress, so that the ultra-low loss fibers with low transmission attenuation can be manufactured. A preparation method of the ultra-low loss optical fibers, disclosed by the invention, is based on the traditional VAD deposition process. A small amount of alkali metal is doped in the deposition process, and the doping amount is small and the gas flow rate is small, so that the normal deposition process is not affected too much, and doping is finished while depositing, and therefore, the productioncycle is prolonged, and then the production stability can be ensured, the production process is not complex, and the preparation method can be used in large-scale production. By the adoption of the invention, optical fiber attenuation can be optimized to ultra-low loss standards, and relay stations can be reduced in high speed transmission with long distance and low attenuation to lower the cost and improve the transmission quality.