<p>A nonlinear constitutive model, based on a parallel rheological framework, is proposed to enhance the characterization of the mechanical properties of composite solid propellants. The mechanical response of HTPB propellant is divided into an equilibrium response and a viscous response through cyclic stretch-relaxation experiments. The equilibrium response incorporates the molecular chain elongation response using the extended eight-chain model, while the cross-linking response in the molecular chain network is described by the tube model. The viscous response is determined by the relaxation rheology of the molecular chains. By comparing with propellant cyclic tensile-relaxation tests, as well as tensile and relaxation test results, it is evident that the developed constitutive model effectively captures the nonlinear mechanical behavior of HTPB propellants. These findings establish a robust basis for accurately characterizing propellant mechanical properties and offer a mechanism for swiftly predicting and analyzing propellant mechanical responses.</p>