The crystal characteristics of decreased sensitivity RDX (D-RDX) and commercial RDX (RDX) were studied by optical microscopy with matching refractive index, scan electron microscopy (SEM), sink-float process and laser particles apparatus. It is found that there was obvious difference of crystal characteristics such as microstructure of inter crystalline voids, particle size and distribution, shape, morphology between D-RDX and commercial RDX. The crystal characteristics value of D-RDX were measured: average particle size being 283μm, the span of size distribution being 0.8, spherical degree being 0.85, smooth particle surface, explosive particles apparent density being above 1.798 g·cm^-3. The shock wave sensitivities of D-RDX and commercial RDX were measured using small scale-gap test, from which the decrease level of the shock wave sensitivity of D-RDX melted with TNT is 10%-15% and casted with HTPB is 31.4%. The difference of shock wave sensitivity of melted or casted explosive charge relates closely to the crystal characteristics of RDX.