In order to obtain castable explosive with high energy and high mechanical strength, CL-20/DNAN/TNT castable explosive was successfully prepared by casting process with 2,4-dinitroanisole (DNAN) and trinitrotoluene (TNT) as low eutectic carriers and hexanitrohexaazoisowurtziane (CL-20) as high energy component. The effects of micro-nano CL-20 particle grading and three functional assistants of N-methyl-4-nitroaniline, tris-(2-chloroethyl) phosphate and catechol on the properties of CL-20/DNAN/TNT castable explosives were investigated. The prepared CL-20-based castable explosives were characterized by SEM, viscosity, density, XRD, mechanical sensitivity, mechanical properties and detonation speed. When the mass ratio of bulk CL-20 to 100 nm CL-20 was 70:30 and 0.5% tri-(2-chloroethyl) phosphate was added, the castable explosive had smooth surface, no obvious internal defects and good density uniformity. Compared with the castable explosive with coarse CL-20, the impact and friction sensitivity was reduced by 32.7% and 57.1%, respectively. The compressive and tensile strength was increased from 7.93 MPa and 3.48 MPa to 33.74 MPa and 4.94 MPa, respectively. The detonation speed was increased by 37.0 m·s^-1 from 8188 m·s^-1 to 8225 m·s^-1.