The preparation of polyurethane/poly(bismaleimide-urethane bismaleimide) [PU/P(BME-UBMI)] IPNs were carried out in two steps. The two-network interaction and the morphology of PU/P(BMI-UBMI) LPNs were characterized by FTIR and TEM. The effect of the interpenetration of P(BMI-UBMI) and PU on the hydrogen bonds of N-H and the pyrolysis procedure of PU/P (BMI-UBMI) IPNs were investigated by DSC and TG. The results indicate that the interpenetration of P(BMI-UBMI) and PU reinforces the hydrogen bonding of N-H, and the incorporating of UBMI enhances the compatibility of the two networks, leading to ideal interpenetrating, which weakens the force constant of C-N-C in maleimide rings. The morphology of PU/P(BMI-UBMI) IPNs shows multiphase domains which have an obvious phase interface. The domains are connected to each other and the domain dimensions are far smaller than those of PU. The thermal weight loss shows obvious stages at different pyrolysis periods. The thermal stability of PU/P(BMI-UBMI) IPNs are much better than that of PU given the content and crosslink density of P(BMI-UBMI) in a certain range.