In order to overcome the defects of conventional A5 booster,such as low energy density,high sensitivity and difficulty in meeting the safety of weapons and ammunitions requiring high energy density and low sensitivity in modern war,ultrafine A₅ booster was developed by coating ultrafine RDX with stearic acid. The mould powder of ultrafine A₅ booster was acquired by aqueous suspension technology. The FT-IR spectra and stereo scan photograph of ultrafine A₅ booster show: stearic acid is covered on the surface of ultrafine RDX. The experiment of impact sensitivity indicates: its critical height (H₅₀) is 3.97 cm,which is higher than that of conventional A₅ using 2.5 kg drop hammer; while the experiment of shock wave sensitivity also shows: its critical gap thickness (X_R) is 3.43 mm,thinner than that of conventional A₅,the experiment of energy output shows: its steel engraving is 0.241 mm deeper than that of conventional A₅.