The invention relates to an iron chimney inner cavity cleaning device and an application method thereof, and belongs to the technical field of chimney cleaning. Firstly, the whole cleaning device is placed in a to-be-cleaned iron chimney from the top of a chimney, after electrification, electromagnet sliding wheels generate magnetism, and the whole device is hung in an iron chimney inner cavity through magnetic force attraction; the whole device moves at a constant speed from top to bottom under the action of gravity of the device, and in the moving process, a cleaning agent is sprayed into the iron chimney inner cavity through a cleaning agent spraying port; meanwhile, a universal rotation bearing rotates and evenly sprays water in a water tank into the iron chimney inner cavity; and water flow is utilized for thoroughly flushing dirt obtained after cleaning through the cleaning agent on the inner wall of an iron chimney, and then the iron chimney inner cavity can be cleaned. The iron chimney cleaning device is easy and convenient to operate, outer traction is not needed, the chimney inner cavity is automatically cleaned, automatic adjustment is conducted so that the iron chimney inner cavity cleaning device can adapt to changes of chimney inner diameters, raise dust is avoided, and wide application prospects are achieved.