The invention discloses a method for preparing anti-pathogenic active oleanane glycoside through chemical conversion. Oleanane glycoside plants of soapberry fruits and the like are taken as raw materials, directed hydrolytic conversion on a crude saponin extract solution through alcohol leaching extraction by using diluted acid or diluted alkali, or using the diluted alkali after the diluted acid or using the diluted acid after the diluted alkali and non-antibacterial active saponin is converted into active oleanane triglycogenin; and then anti-pathogenic active oleanane glycoside with high concentration is obtained through macroporous resin adsorption, washing for acid, alkali or salt removal and alcohol washing on the active oleanane triglycogenin. The prepared saponin has prevention and control activity on various crop pathogenic fungi of rice blast and the like, and the effective concentration in the antibacterial process is greatly reduced in comparison with that of mixed saponin of raw materials.