The invention discloses an aluminum oxide microwave dielectric ceramic and a preparation method thereof. The aluminum oxide microwave dielectric ceramic comprises aluminum oxide, yttrium oxide, zirconium oxide and a CMS mixture, wherein the mass percent of aluminum oxide is 95%-98%, the mass percent of yttrium oxide is 0.25%-0.75%, the mass percent of zirconium oxide is 0%-3%, and the mass percent of the CMS mixture is 1.25%-3.25%; the CMS mixture comprises magnesium oxide, calcium oxide and silicon oxide, wherein the mass percent of magnesium oxide is 18.6%, the mass percent of calcium oxide is 50.1%-56.0% and the mass percent of silicon oxide is 25.4%-31.3%. The preparation method comprises: mixing aluminum oxide, yttrium oxide, zirconium oxide and the CMS mixture in proportion, performing ball-milling and pelleting, forming blank pieces by pressing, performing rubber discharging and sintering to prepare the ceramic. By adding the combined oxide, the ceramic is endowed with high microwave dielectric properties and high breakdown strength.