The ultrafine RDX explosives are prepared by the method of pulse ram-type pulverization,and the RDX particles with 4.020 μm are obtained at 120 MPa impulse pressure and RDX/water in the mass ratio of 1：5. The effect of water propotion,impulse pressure and cycle times are discussed as well,and the results indicate that the particle size of RDX decreases with increase of water proportion and impulse pressure. And to some extent,the particle size of RDX decreases slightly with increase of cycle times.