Interpenetrating polymer networks (IPNs) were prepared from polyurethane (PU)-modified epoxy with different molecular weight of polyol and polyurethanes based on the mixture of polydiol and polytriol by a one-shot method. Two types of PU-modified epoxy: PU-crosslinked epoxy and PU-dangled epoxy were synthesized, and the effects of the different molecular weights of polyol in the PU-modified epoxy/PU IPNs on the dynamic mechanical properties, morphology, and damping behavior were investigated. The results show that the damping ability is enhanced through the introduction of PU-modified epoxy into the PU matrix to form the IPN structure. As the molecular weight of polyol in PU-modified epoxy increases, the loss area (LA) of the two types of the IPNs increases. PU-dangled epoxy/PU IPNs exhibit much higher damping property than that of the PU-crosslinked epoxy/PU IPNs with 20 wt % of PU-crosslinked epoxy.