NOVELTY - A biological oil is prepared by pulverizing, drying and sieving biomass to prepare into solid particles and feeding biomass particles and inert gas into rotary reactor; controlling temperature of a high frequency induction heating system at 450-600 degrees C to thermally crack the biomass in rotary type reactor; adhering generated tar on rotary type reactor, and conveying other product generated by the reaction to cyclone separator; and separating conveyed other products into oil-gas mixture and carbon.    USE - Method for preparing biological oil (claimed).    ADVANTAGE - The method has improved heating efficiency, reduces preparation cost of bio-oil, and is simple and easy to operate.    DETAILED DESCRIPTION - Biological oil preparation comprises using high frequency induction heating by a device comprising spiral conveyer, rotary reactor, high frequency induction heating system, cyclone separator and condenser. An outlet side of the spiral conveyer is connected with a side of upper part of the rotary reactor. An upper part face of the spiral conveyer has hopper far from the rotary reactor. An end face of an end of the spiral conveyer far from the rotary reactor has air inlet pipe. An outer wall of the rotary reactor has the high frequency induction heating system that can automatically control heating temperature via a temperature control system. The rotary reactor is connected with a side of the upper part of the cyclone separator via a product conveying pipe at its top end face. The bottom of the rotary reactor has tar outlet. The cyclone separator is connected with the top end face of the condenser via an oil gas conveying pipe at its top end face. The bottom of the cyclone separator has carbon outlet. A top end face of the condenser has fuel gas outlet and its bottom has biological oil outlet. The method comprises (A) pulverizing, drying and sieving biomass to prepare into solid particles of 35-50 meshes, respectively feeding the biomass particles and inert gas into rotary reactor through the hopper and air inlet pipe of the spiral conveyer; and when conveying, controlling feeding speed of the biomass particles to be 40-50 kg/hour, air feeding speed of the inert gas to be 20-25 m/s and maintaining pressure value of the hopper at 0.02-0.1 MPa; (B) controlling temperature of the high frequency induction heating system at 450-600 degrees C to thermally crack the biomass in rotary type reactor; (C) adhering generated tar on rotary type reactor, and conveying other product generated by the reaction to the cyclone separator, the other product comprising biological oil (30-40%), inert gas (25-35%), non-condensed gas (10-15%), and solid carbon particles; (D) separating the conveyed other products into oil-gas mixture and carbon via the cyclone separator, conveying the separated oil-gas mixture to condenser, and discharging and collecting the separated carbon by the carbon outlet at bottom of the cyclone separator; (E) condensing the fed oil-gas mixture by the condenser to generate biological oil and mixed gas of inert gas and non-condensed gas from the oil-gas mixture, discharging and collecting the obtained biological oil via the biological oil outlet at bottom of the condenser, and discharging and collecting the biomass gas produced by condensation via the fuel gas outlet at the top of the condenser; and (F) when the obtained biological oil and biomass gas are completely discharged and collected by the container, finishing the preparation of biological oil.