Abstract: Coal gasification technology, one of the main ways to clean utilization of coal, has been developed rapidly in China. Coal gasification fine ash is a new type of solid waste inevitably produced in the process of coal gasification. With the implementation of green and clean production and the development of a circular economy, the resource utilization and harmless treatment technology of coal gasification fine ash have become the key to achieving both the environmental and economic benefits of coal gasification technology. Taking coal gasification fine ash as the research object, the samples with a median diameter of 2 μm were prepared by drying and ball milling. The physicochemical properties of the pre-treated coal gasification fine ash were characterized by XRD, Raman, SEM, TEM, and XPS, and the crystal structure, micromorphology, composition, molecular structure, and chemical state of the elements of the coal gasification fine ash were analyzed. Moreover, the electromagnetic wave absorption performance was tested. The results show that the fine ash contains some graphitized carbon and zero-valent iron, and has a relatively complete pore structure, which provides a possibility for it to be used as an electromagnetic wave-absorbing material. The absorbers exhibit the minimum reflection loss (RL_min) of −17.6 dB at 1.7 mm, and broad absorption bandwidth (RL≤−10 dB) of 4.2 GHz. In addition, the radar cross-section simulation results show that the fine ash/paraffin coating can effectively reduce the electromagnetic wave scattering of the perfect conductor substrate.