The invention discloses glass powder of water-based glass ionomer cement and a preparation method thereof, belonging to the field of inorganic non-metal materials. The powder is prepared from the following raw materials in percentage by mass: 25-35% of Al2O3, 30-45% of SiO2, 5-8% of MO, 2-8% of Na2O, 5-10% of ZnO, 3-9% of P2O5 and 5-15% of CaF2, wherein MO is one of SrO and CaO or a mixture of the two. The preparation method of the powder sequentially comprises the following steps: preparing the raw materials, fusing, quenching with water, pulverizing, performing surface treatment, drying, grinding and screening. Compared with the prior art, the invention has the advantages of low cost, stable product quality, fine mechanical comprehensive properties, X ray rejection, continuous fluorion release and uniform particle size distribution, and is suitable for industrial mass production.