The invention provides a method for converting and extracting resveratrol by means of bacteria. A fermentation medium with the addition of a substrate, namely a polygonum cuspidatum crude medicinal material, is inoculated with a bacillus safensis seed liquid, fermentation is conducted at the temperature of 28-37 DEGC for 6-15 hours, when the thalli concentration OD600nm is equal to 0.8-1.6, a product starts to be separated out, thalli, the substrate and the product are interacted, the product with the thalli as crystal nuclei is crystallized and separated out under the action of microorganisms, fermentation liquid is centrifuged, washed with distilled water and precipitated, an organic solvent is added into precipitates, one time of extraction and drying are conducted, and the resveratrol product can be obtained. The purity is larger than or equal to 99%. The method is simple in operation, mild in condition, short in conversion time and high in product yield.