A fan backflow prevention structure includes a frame body, a first static blade assembly and a second static blade assembly. The frame body has a peripheral wall. Two end edges of the peripheral wall are respectively formed as an air inlet and as an air outlet. A base seat is correspondingly disposed at the air outlet. The first static blade assembly extends outward from the base seat and includes multiple static blade sections. Two ends of each static blade section are respectively connected with the base seat and the peripheral wall. The second static blade assembly is disposed at the air outlet and rotatably coupled to the base seat. The second static blade assembly has a connecting section and multiple stop sections extending outward from the connecting section.