Pertaining to the computer modeling for analyzing bullet or fragment impact sensitivities of solid rocket motors, the input parameters of JWL Equation to describe the unreacted and reacted states of solid propellants are inevitable. To obtain the parameters by experiments is complicated and expensive. This work proposes a theoretical method including Rodean＇s theory and the additivity principle, to estimate the unreacted states of composite propellants. First, Rodean＇s theory and the additivity principle are appraised to be utilizable by the experimental data of homogeneous explosives and composite explosives; then they are applied to estimate the unreacted states of two composite propellants. The theoretical estimations for the unreacted states of the composite propellants are consistent with the experimental results depicted in the literature. With the Marquardt＇s nonlinear modeling method, the estimated states can be modeled into JWL Equation for gaining the relative parameters. In addition, the modified TIGER code is applied to evaluate the reacted states of one nitramine composite propellant and detonation properties of PBXW-115.