The chemical composition and morphology properties of high-aluminum fly ash were studied by means of XRF, XRD, SEM, and EDS. It was found that silicon in high-aluminum fly ash mainly existed as mullite and amorphous SiO2 with hardly any quartz, and both the surface of fly ash particles and the smaller fly ash particles attached on the larger ones were enriched in silica. The desilication process was used to remove the amorphous SiO2 from high-aluminum fly ash. The desilication process was carried out using a low concentration of sodium hydroxide at a relatively low temperature. In order to improve the desilication efficiency, pretreatment was added before the desilication reaction. After comparing results of pretreatment reactions under different temperatures, the sample treated at 65 degrees C was analyzed in detail because its desilication effect was the best. After additional pretreatment using the desilication process, the ratio of Al/Si in desilication fly ash was increased from 1.92 to 2.51. The silica content of the filtrate was increased by 33.58%. The reaction mechanism can be explained by the formation of zeolite P, which consumed the unstable mullite of high-aluminum fly ash and blocked the formation of hydroxy sodalite in the reaction during treatment at 95 degrees C.