<p>The fly ash water treatment filler prepared using the foaming method is characterized by low energy consumption and a simple preparation process, effectively promoting the resource utilization of fly ash. To further enhance the filler's water treatment performance, this study employed the co-precipitation method to synthesize Fe₃O₄ magnetic particles and optimize the magnetic modification of the foamed fly ash filler. The results show that the specific surface area of the magnetically modified foamed fly ash filler increased by 30.51%, and the average pore diameter increased by 26.47% compared with the original filler before modification. In the simulated domestic wastewater treatment test, the magnetically modified filler achieved COD and ammonia nitrogen removal rates of 74.61% and 70.18%, respectively. These values are higher than those of the unmodified and sintered ceramic fillers and represent improvements of 21.99% and 9.76% over the unmodified filler. Illumina MiSeq sequencing revealed that the microbial community on the magnetically modified filler was more abundant, with <i>Proteobacteria</i> and <i>Bacteroidetes</i> as the dominant phyla. Magnetic modification is more effective in promoting the formation of biofilms on the surface of fillers, enhancing microbial diversity, and increasing the proportion of functional bacteria.</p>