A detachable USB fan module mounting structure includes a machine case having a top opening and two guide rails at two opposite lateral sides of the top opening, a circuit board with one a USB connector accommodated in the machine case, a cover member detachably covered on the top opening of the machine case and having a coupling structure detachably coupled to the guide rails of the machine case and a bottom bracket with a fan module positioning structure to hold a USB fan module that has a power cord with a USB plug connected to the USB connector of the circuit board.