The invention discloses magnesium oxysulfate cement and a preparation method thereof. The preparation method comprises the following steps: mixing and grinding 50 to 100 parts by mass of active magnesium oxide powder, 30 to 100 parts by mass of crystalline magnesium sulfate or anhydrous magnesium sulfate, 0.5 to 2 parts by mass of an additive and 0.5 to 10 parts by mass of a basic magnesium sulfate precursor and then carrying sieving with a 100-to-200-mesh screen so as to obtain the rapid-hardening magnesium oxysulfate cement. According to the invention, the speed of setting and hardening is substantially increased, and production efficiency of the magnesium oxysulfate cement can be improved.