A novel interpenetrating polymer network, which based on modified hyperbranched polyester and HTPB polyurethane, was prepared for enhancing mechanical properties of HTPB based composite solid propellants. The results indicate that with the modified hyperbranched polyester added, the tensile strength and elongation of HTPB polyurethane elastomer improves obviously. Specially, when the content of modified hyperbranched polyester of generation 4 rises up to twenty percentage of the whole weight, the resultant elastomer has the best tensile strength and elongation, which increase 3.36 times and 3.69 times than pure elastomer, and reach 2.3 MPa and 1860% respectively. Therefore, the modified hyperbranched polyester can serve as a better modifer for HTPB propellants.