Silicon carbide (SiC)-alumina (Al2O3)-mullite (Al6Si2O13) composite powder was successfully synthesized at 1 550 ℃ for 5 h via carbothermal reduction reaction, and the effects of various mass ratios of active carbon to fly ash (0.38, 0.44 and 0.58) on the phase composition and microstructure of products were investigated, and the formation process of the powder was also analyzed in detail. The products mainly consist of β-SiC, α-Al2O3, Al6Si2O13 and FeSi. Increasing carbon content favors the decomposition of Al6Si2O13 and formation of SiC. The average particle size of β-SiC and α-Al2O3 is about 1 μm and that of Al6Si2O13  is 5-10 μm. The formation process of SiC-Al2O3-Al6Si2O13 powder includes the decomposition of mullite in fly ash and formation of SiC.