The invention provides a preparation method of an active substance capable of inhibiting plant viruses and plant bacteria and application of the active substance. The preparation method of the active substance capable of inhibiting plant viruses and plant bacteria comprises that folium artemisiae argyi in Guizhou province is taken as raw material, dried in the sun and then smashed, ethanol extraction is carried out; ethanol extract is extracted by virtue of trichloromethane; reduced pressure concentration is carried out on a trichloromethane extraction layer, so that crude extract is obtained; silica column chromatography is carried out on trichloromethane crude extract, gradient elution is carried out by virtue of petroleum ether-acetic acid ethyl ester mixed eluant in different ratios, then purification means like the silica column chromatography is carried out for a plurality of times, and two monomer compounds, namely 8 alpha-hydroxyl artemisia terpene and 5,4'-dihydroxyl-6,7,3'-trimethoxy flavone, are obtained. Biological activity tests show that the monomer compound I has good prevention and control effect on plant viruses such as cucumber mosaic virus, tobacco mosaic virus, south rice black-streaked dwarf virus and rice stripe virus and 5,4'-dihydroxyl-6,7,3'-trimethoxy flavone has good inhibiting effect on bacterial disease pathogenic bacteria such as tomato bacterial wilt, tobacco bacterial wilt, rice bacterial leaf blight and rice bacterial leaf streak.