A cooling fan structure includes a housing, a multistage fan wheel, and a stator-blade unit set. The housing is integrally formed and includes a first and a second frame, and a connecting spacer set connected to between the first and the second frame to define an open space therebetween. The multistage fan wheel is located in the housing and has a hub, around which a group of first and a group of second rotor blades are formed and axially spaced to define a bladeless zone on the hub between the first and the second rotor blades. The stator-blade unit set includes a first and a second stator-blade unit, which are assembled into the open space of the housing to circumferentially close the open space. The stator-blade unit set is exchangeable according to different high impedance systems to achieve different levels of high static pressure without increasing any mold-making cost.