A method for preparing allicin injection comprising the following steps: first step, extracting allicin, which comprises the following steps: adding alliinase and deoxygenated pure water to a container filled with argon or high purity nitrogen in water-to-alliinase ratio of 1000~2000 ml: 35.4g, stiring thoroughly to completely dissolve the alliinase, and adjusting the temperature of water bath to 25-35°C and controlling pH at 6.5-8.5, to obtain an alliinase solution; preparing an alliin solution in water-to-alliin ratio of 500~1000 ml: 35.4g, and dropwise dropping the alliin solution into the container containing the alliinase solution, stirring thoroughly and filling continuously the container with argon or high purity nitrogen so that the container is fully filled with the argon or high purity nitrogen, wherein a ratio of alliin to alliinase is 1 mg: 1~2 IU; and after completion of the dropwise dropping the alliin solution, continuously stirring and reacting for 20 to 40 minutes in total to obtain an enzymolysis reaction solution containing allicin, and then immediately replacing warm bath water with ice water to lower down the temperature of the water bath (containing excess ices) to 0°C, and using an ultrafiltration membrane of 1000~5000Da to remove alliinase and obtain an aqueous solution of allicin; second step, diluting the allicin obtained in the first step with a solvent precooled to a temperature of 0-4°C within a clean bio-safety cabinet, to obtain an allicin solution; encapsulating the allicin solution into an ampoule filled with nitrogen or argon to obtain the allicin injection, which is used to treat septicemia or tumors by intravenously guttae. first step, extracting allicin, which comprises the following steps: adding alliinase and deoxygenated pure water to a container filled with argon or high purity nitrogen in water-to-alliinase ratio of 1000~2000 ml: 35.4g, stiring thoroughly to completely dissolve the alliinase, and adjusting the temperature of water bath to 25-35°C and controlling pH at 6.5-8.5, to obtain an alliinase solution; preparing an alliin solution in water-to-alliin ratio of 500~1000 ml: 35.4g, and dropwise dropping the alliin solution into the container containing the alliinase solution, stirring thoroughly and filling continuously the container with argon or high purity nitrogen so that the container is fully filled with the argon or high purity nitrogen, wherein a ratio of alliin to alliinase is 1 mg: 1~2 IU; and after completion of the dropwise dropping the alliin solution, continuously stirring and reacting for 20 to 40 minutes in total to obtain an enzymolysis reaction solution containing allicin, and then immediately replacing warm bath water with ice water to lower down the temperature of the water bath (containing excess ices) to 0°C, and using an ultrafiltration membrane of 1000~5000Da to remove alliinase and obtain an aqueous solution of allicin; adding alliinase and deoxygenated pure water to a container filled with argon or high purity nitrogen in water-to-alliinase ratio of 1000~2000 ml: 35.4g, stiring thoroughly to completely dissolve the alliinase, and adjusting the temperature of water bath to 25-35°C and controlling pH at 6.5-8.5, to obtain an alliinase solution; preparing an alliin solution in water-to-alliin ratio of 500~1000 ml: 35.4g, and dropwise dropping the alliin solution into the container containing the alliinase solution, stirring thoroughly and filling continuously the container with argon or high purity nitrogen so that the container is fully filled with the argon or high purity nitrogen, wherein a ratio of alliin to alliinase is 1 mg: 1~2 IU; and after completion of the dropwise dropping the alliin solution, continuously stirring and reacting for 20 to 40 minutes in total to obtain an enzymolysis reaction solution containing allicin, and then immediately replacing warm bath water with ice water to lower down the temperature of the water bath (containing excess ices) to 0°C, and using an ultrafiltration membrane of 1000~5000Da to remove alliinase and obtain an aqueous solution of allicin; second step, diluting the allicin obtained in the first step with a solvent precooled to a temperature of 0-4°C within a clean bio-safety cabinet, to obtain an allicin solution; encapsulating the allicin solution into an ampoule filled with nitrogen or argon to obtain the allicin injection, which is used to treat septicemia or tumors by intravenously guttae.