To investigate the mechanism of toughening and reinforcement, ATR-FTIR and TEM were used to analysis chemical and morphology structures of HBPE/HTPB polyurethane interpenetrating polymer network (IPN). Results show H-bond between the two network and phase separation enhance the mechanical properties of IPNs. When the HBPE is used as dispersed phase and its domain size is about 1 μm, the mechanical properties of IPN reach the maximum.