A cooling fan includes a sidewall coupled to a base of a housing and defining a compartment. The housing includes an air inlet, an air outlet, and a dust channel. The air inlet, the air outlet, and the dust channel are in communication with the compartment. An impeller is rotatably coupled to a stator coupled to the base. A control element includes a driving circuit electrically connected to the stator and a rotating direction control circuit electrically connected to the driving circuit. The air outlet and the dust channel separate an inner periphery of the sidewall into first and second guiding wall sections. The first guiding wall section has a guiding portion contiguous to the dust channel. A first spacing between the guiding portion and a center of the impeller is not larger than a second spacing between the second guiding wall section and the center of the impeller.