The invention discloses a Polygonum cuspidatum root endophytic strain. The strain is named Penicillium oxalicum G3 in a classified way, the strain was collected in China General Microbiological Culture Collection Center on January 17, 2011, and the collection number is CGMCC NO.4558. Polydatin in Polygonum cuspidatum can be transformed into resveratrol by inoculating the strain into a culture medium containing Polygonum cuspidatum boiling extracting solution, so that the polydatin can be directly transformed into the resveratrol through microbial fermentation, the extraction of the polydatin is not required, the cost can be saved, and the pollution to the environment can be reduced.