The invention relates to a method for preparing in-situ particle reinforced magnesia-based composite materials, which particularly comprises the following steps: magnesium aluminum alloy raw materials are completely smelt under the protective condition of gas mixture of SF6 and CO2; aluminum-silicon intermediate alloy is pressed into a magnesium aluminum fused mass, and the temperature is raised and maintained, so as to guarantee complete reaction between silicon and magnesium in the magnesium aluminum alloy fused mass, and simultaneously an ultrasonic probe is inserted into the fused mass for treatment; and after the treatment is finished, surface scum is removed and the fused mass is cast into a metal die, and the in-situ Mg2Si particle reinforced magnesia-based composite materials are obtained. The method has the characteristics of good interference compatibility of Mg2Si particles and substrates, controllable particle size of reinforced phases, uniform distribution of the reinforced phase and so on; and the magnesia-based composite materials prepared have good mechanical property.