Abstract: There are some problems in the traditional way to determine a relaxation modulus by a static relaxation test. Based on Joonas Sorvari’s work, an improved way, named improved Sorvari way, was presented. The process to gain a relaxation modulus by this way was introduced. The modified DB propellant’s relaxation test was done, and the relaxation modulus were obtained by the improved Sorvari way. The linear viscoelastic constitutive was then set up to predict a stress relaxation process. The results show the improved Sorvari way is better than traditional way and Sorvari’s way, and the improved Sorvari way is suitable for determining modified DB propellant’s relaxation modules.