The invention relates to a method for preparing nanometer CaCO3 from calcium carbide slag through an ultrasonic process. The calcium carbide slag is purified to generate a Ca(OH)2 emulsion, the Ca(OH)2 emulsion which is added with a dispersant, CO2 and air is irradiated by ultrasonic waves, uniformly stirred, suck-filtered and dried to generate the nanometer CaCO3. The method for preparing the nanometer CaCO3 through adopting the ultrasonic process has the advantages of simple operation, no need of designing a new device, high product added value, gas-liquid-solid mass transfer speed improvement benefiting, realization of the resource comprehensive utilization of the industrial waste slag calcium carbide slag, limestone resource consumption reduction, production cost reduction, natural environment improvement, novel technology, operation benefiting, good product stability, substantial shortening of the preparation time, environment improvement, particle size of 30-200nm, and purity of above 99.2%. Production requirements of the rubber industry, the plastic industry and the coating industry are completely met.