In order to provide the thermodynamic criterion for the cocrystal formation and give theoretical basis for the screening and optimization of crystallization solvents and parameters, the thermodynamic of hexanitrohexaazaisowurtzitane (CL-20)/1,4-dinitroimidazole (1,4-DNI) cocrystal formation has been investigated. Taking acetone, ethyl acetate and methanol as solvents, the solubility data of CL-20, 1,4-DNI in pure solvents and CL-20 in 1,4-DNI solutions with different concentrations (0.04, 0.06, 0.08, 0.10, 0.12, 0.16, 0.20, 0.24 g·mL^-1) were achieved with the help of high performance liquid chromatography. The ternary phase diagrams of CL-20/1,4-DNI solvents were built. The thermodynamic parameters of the solubility product K_sp, the complexation constant K₁₁, and the reaction free energy ΔG⁰ were obtained through fitting the solubility data based on the mathematical models of solution chemistry theory. The results show that if the solubilities of CL-20 and 1,4-DNI are larger and their discrepency is less in a solvent, the cocrystal region is larger and its shape is more symmetrical. The order of the cocrystal regions in the three solvents from large to small is acetone > ethyl acetate > methanol, and the shape of the cocrystal region in the acetone and ethyl acetate is more symmetrical. The calculated thermodynamic parameters indicate that the acetone is the most beneficial to the formation of CL-20/1,4-DNI cocrystal among the three solvents, followed by ethyl acetate. The ternary phase diagram and thermodynamic parameters in acetone solvent at different temperatures show that lowering the crystallization temperature is beneficial to the precipitation of the CL-20/1,4-DNI cocrystal.