The invention discloses modified fly ash and a preparation method thereof, which relates to fly ash and a preparation method thereof. The invention aims to solve the problems of poor gelling property of wet discharged fly ash after being dried and energy waste during the drying. The modified fly ash of the invention is made of the wet discharged fly ash and an active agent in weight proportion of 1.2-2.5: 1. The active agent is the combination of quicklime powder and one or a plurality of gesso, sodium hydroxide or powdery sodium silicate. The preparation method of the modified fly ash is that: the wet discharged fly ash is naturally dried till the moisture content thereof is 17-20 percent, then the active agent is added, and the mixture is stirred till the moisture content thereof is less than 1 percent; the mixture is ground with ball mill till the fineness reaches 45Mu and sieve residue 5-12 percent. The modified fly ash of the invention has gelling property and the compressive strength thereof exceeds 10 MPa after 28 days. The invention has the advantages of simple process, and resource and energy saving.