The vacuum oxidation crystallization process was applied to the preparation of RDX by a direct nitration method, which effectively solved the problem of a large amount of waste acid produced in the direct nitration. The optimum process conditions were experimentally investigated by a single factor method. Results show that optimum process conditions are determined as: with 1 g HA as a reference, in the process of oxidation, the amount of dilution water is 3.51 g, the lowest excited temperature 49.4 ℃, heating temperature 60 ℃ and the adding rate of dilution water amount 0.12 g·s^-1 and in the process of vacuum crystallization, vacuum tightness is 82 kPa, initial evaporation temperature 85.0 ℃, stirring speed 650 r·min^-1, forming 10.34 g of waste acid. Compared with the traditional process, the new process makes the amount of dilution water and waste acid reduce 24.9% and 10.2%, respectively.