The prestrain influence on viscoelastic properties $|E^\star|$ and $\tan \delta$ of four different HTPB composite propellants was measured using Dynamic Mechanical Analyses (DMA). A nonlinear behaviour in terms of prestrain was observed and then modelled using a modified generalized Maxwell model. Prestrain was introduced as a variable of the stiffness of each Maxwell element using simple relations with two parameters. An algorithm was proposed and numerically implemented to identify the model parameters from the measurements. The performance of the identification method is discussed in terms of accuracy of predictions when compared with measurements and in terms of robustness of the involved parameters. The good match with experimental measurements shows that the dependence on the uniaxial prestrain of the complex modulus of the studied composite propellants can be described with few Maxwell elements and yet good accuracy, for small prestrains.