Abstract: For reducing uranium conversion producing cost, raising the activity of UO₃ is an important measure. Activated UO₃ was prepared using uranyl nitrate solution as raw material, the high temperature gas produced by propane combustion as heat source, in the self-built denitrate equipment by means of high-pressure spray method. The effects of the uranium concentration in acid uranyl nitrate solution, reaction temperature and operation pressure on the activity of UO₃ were investigated. Furthermore, the effect of the pressure nozzle structure of atomizer on UO₃ particle size distribution was analyzed. The results show that the activity of UO₃ is fine. High specific surface area of 18 m²/g and the particle sizes between 15 μm to 50 μm of UO₃ powder are achieved at the reaction temperature of 400 ℃ and operation pressure of -100 Pa. The reason is that liquid jet atomization process in air stream with high temperature and high speed cross flow interacts. There is not only the mechanism of liquid jet atomization tearing by using the air stream with high temperature and high speed, but also the mixture boil, resulting in the increase of the specific surface area and the porosity of the product. The method of the preparation of activated UO₃ has potential ability in industry.