In order to reduce the sensitivity of hexanitrohexaazaisowurtzitane (HNIW), the waterborne polyurethane grafted styrene and acrylonitrile copolymer (WPU-g-SAN) were synthesized from isophorone diisocyanate (IPDI), 2-hydroxyethyl methacrylate (HEMA), styrene (S) and acrylonitrile (AN) as raw materials by seeded emulsion polymerization in situ. The cast films obtained from WPU-g-SAN were characterized by Fourier transform infrared (FTIR), wide-angle X-ray diffraction (WAXD), nuclear magnetic resonance (¹H NMR) and dynamic mechanical analysis (DMA). The results showed that the title copolymer was amorphous and exhibited a certain degree of phase separation. HNIW and HNIW coated with the WPU-g-SAN were examined by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The impact sensitivity was tested by method of GJB772A-1997. The experimental results indicated that the characteristic height H₅₀ of HNIW coated with WPU-g-SAN increases from 13.6 cm to 28.5 cm.