NOVELTY - Preparing artemisinin, comprises (i) naturally drying the entire Artemisia annua, crushing, and sieving; and (ii) crushing Artemisia annua grass, sieving, loading into extraction kettle, and extracting, taking out carbon-dioxide, passing into carbon-dioxide tank, cooling, transferring to pre-heater, transferring the raw material based on mass transfer, dissolving a solute of supercritical carbon-dioxide by a two-stage heating level, and adding into a separate kettle I and a separate kettle II, and recycling carbon-dioxide by carrying out two level vacuum drying process.    USE - The method is useful for preparing artemisinin (claimed).    ADVANTAGE - The method is environmental friendly, economical, and suitable for large-scale industrial production; utilizes supercritical fluid as liquid solvent which not only has the ability to extraction on equivalent basis, but also facilitates mass transfer effect; prevents extensive utilization rate of organic solvents, and human health hazards caused during the extraction process; shortens the extraction time; has high extraction rate; and saves energy.    DETAILED DESCRIPTION - Preparing artemisinin, comprises (i) naturally drying the entire Artemisia annua, crushing, and sieving over 40-80 meshes; and (ii) crushing yellow Artemisia annua grass, and sieving, and loading into an extraction kettle of 1 l supercritical extraction device, and extracting, taking out carbon-dioxide, and passing into a carbon-dioxide tank, cooling into a liquid state, transferring into a pre-heater of the extraction kettle by a high pressure pump, and transferring the raw material based on mass transfer, dissolving a solute of supercritical carbon-dioxide by a two-stage heating level, and adding into a separate kettle I and a separate kettle II under two-stage pressure conditions, and recycling the carbon-dioxide obtained from top of the separate kettle II by carrying out two level vacuum drying process, and circulating in a cooling system, and controlling the hot water at constant temperature in the extraction kettle and separation kettle, where the extraction pressure is 15-25 MPa, two separate reactor volume is 0.6 l, and extraction temperature is 30-70 degrees C, carbon-dioxide mass flow is 1-2 kg/hour, where high pressure supercritical carbon-dioxide fluid is used as an extraction fluid. The mixing process is carried out based on isothermal and isobaric rules. The separation process is carried out by countercurrent pressurization and vacuum evaporation processes, where the separated Artemisia annua-contains peroxide bridge sesquiterpene lactones.