The invention provides biomass base oil which is prepared from a natural fatty alcohol front cut fraction as a raw material. The invention provides a preparation method of the biomass base oil. The preparation method comprises the following step: under the action of a catalyst, enabling the natural fatty alcohol front cut fraction, an alkyl reagent and an alkaline compound to react, so as to obtain the biomass base oil. The biomass base oil provided by the invention is a natural aliphatic hydrocarbon mixed ether, has the characteristics of environmental protection, no toxicity, strong lubrication, no fluorescence, high-temperature resistance, salt resistance, calcium resistance and hydrolysis resistance, and is base oil with excellent performance for lubricants. The preparation method of the biomass base oil provided by the invention has a high reaction yield, good product quality, simple preparation processes and mild conditions, and the preparation process is non-toxic, efficient andenvironmental-friendly.