The invention discloses a preparing method for a cambiform-magnesium-carbonate-coated-calcium-carbonate composite material, and belongs to the technical field of material-chemistry inorganic nonmetallic materials. The preparing method of the composite material includes the following steps that desulphurization gypsum serves as a calcium source, and is reacted for 5 hours to 12 hours in a 0.5 M-6.0 M hydrochloric-acid solution system at the temperature of 100 DEG C to 160 DEG C and filtered, then filter liquid is recrystallized through ethyl alcohol, the recrystallized filter liquid is dried at the temperature of 55 DEG C to 85 DEG C, and a pure calcium-sulfate product is obtained; magnesium acetate serves as a magnesium source, and is reacted with the obtained calcium-sulfate product for 8 hours to 24 hours in a urea solution system at the temperature of 150 DEG C to 180 DEG C, ethyl alcohol serves as leaching liquid, washing is carried out at the room temperature, filtering is carried out, drying is carried out at the temperature of 55 DEG C to 85 DEG C, and the cambiform-magnesium-carbonate-coated-calcium-carbonate composite material is obtained. The method is simple in synthetic route and convenient to operate, secondary utilization of chemical-industry-waste desulphurization gypsum can be achieved, and the product can serve as an additive in the rubber industry, the plastic industry and the like, and can also serve as a preparing raw material of a heat-preservation fireproofing material.