A fan device capable of increasing air pressure and air supply includes a frame, a first blade wheel, a second blade wheel and a guide flow part. The frame is a hollow casing to define a flow passage with a first air port and a second air port available for air passing through. The guide flow part is held with a support in the flow passage and a driving device is received in the support. The first blade wheel with first fan blades and the second blade wheel with second fan blades are disposed in the flow passage next to the support for being driven by the driving device. The number of the first fan blades is more than that of the second fan blades. Once the first and second fan wheels are in operation, air induced by the fan wheels is guided by the guide flow part and air pressure and air supply of the fan system are increased substantively due to number of the first fan blades being greater than that of the second fan blades.