The invention provides a preparation method for ultramicro scandium oxide. The method comprises the following steps: extracting a scandium-containing ionic solution by adopting a mixed solution of a carboxylic acid extraction agent and an organic solvent to obtain a scandium-loaded organic phase, wherein the carboxylic acid extraction agent in the mixed solution of the carboxylic acid extraction agent and the organic solvent is added excessively relative to the scandium ions in the scandium-containing ionic solution; mixing the scandium-loaded organic phase with ammonia water and carrying outa saponification reaction on to form a water-oil emulsion; heating and drying the water-oil emulsion and carrying out a thermal hydrolysis reaction to obtain a scandium precipitate; calcining the scandium precipitate to obtain the ultramicro scandium oxide. According to the preparation method, a scandium oxide product of an ultramicro scale in dimension is prepared through extraction by means of the carboxylic acid extraction, and control of shape and dimension of the scandium precipitate by a microreactor in combination with acid and alkali adjusting capability of ammonia water and coating capacity of the carboxylic acid extraction agent in the thermal hydrolysis reaction process. Besides, the preparation method provided by the invention also has the characteristics of being simple and efficient and is more favorably applied to industrial production.