A positive-negative-rotation fan for a clothes dryer comprises multiple whole-size fan blades arranged on a periphery of an outer frame of the fan, and multiple half-size fan blades arranged on the periphery of the outer frame of the fan. The half-size fan blades and the whole-size fan blades have similar shapes, and the size of an edge of each half-size fan blade parallel to a radial direction of the fan is half of the size of a corresponding edge of the whole-size fan blade. The clothes dryer using the positive-negative-rotation fan can provide an even and great air volume and a great air intake pressure in a clockwise and counterclockwise alternating rotation process, and accordingly clothes are evenly dried and the drying time is short. By using a flow guide device, aggregated back-flows are not generated between the fan and an outlet of a volute.