To solve the problems of the traditional production process of ultrafine nitroguanidine (NQ), such as a long process and easy pollution caused by recrystallization, the ultrafine NQ (2−6 μm) was prepared by the one-step crystallization in an impinging stream-rotating packed bed (IS-RPB). The influence of operation parameters on the size of the NQ crystal was discussed through various particle size characterization methods. Results show that the size of ultrafine NQ prepared in the IS-RPB was 2.0−3.0 μm, which was smaller than that of market products and conformed to the military standard. Simultaneously, the crystal morphology tended to reduce the length, meeting the target requirement of "shorter and thicker" high-end products. The role of high dispersion and strong mixing in the IS-RPB was the key to continuous and one-step nucleation and crystallization of NQ. One-step crystallization in the IS-RPB is a push flow operation process, where abundant accumulation of the raw material will be avoided effectively, and the safety of energetic material processing will be enhanced greatly. Ultrafine NQ can be prepared without recrystallization, which greatly reduces the energy consumption and the pollution of organic impurities to the product and the environment, providing a new way for the continuous production of ultrafine NQ.