To address the problems of narrow damping temperature range and unclear performance regulation mechanism of traditional EP/PU IPNs materials, polyurethane prepolymers were prepared using 4,4'-diphenylmethane diisocyanate (MDI-50) and polypropylene glycol (PPG-2000), and then copolymerized with E-51 epoxy resin to construct an interpenetrating polymer network (IPNs) system, followed by curing via the MOCA process. By adjusting the block length of the prepolymers, a series of polymer materials with both mechanical and damping properties were designed. The effects of prepolymer block length on the thermal stability and phase structure of the materials were investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy and atomic force microscopy. The influences of prepolymer block length on the mechanical and damping properties were characterized by universal testing machine, impact tester and dynamic mechanical analysis.The results show that with the increase of prepolymer block length, the epoxy resin (EP) phase transforms from a continuous distribution to an isolated "island phase", leading to a gradual decrease in the glass transition temperature of the polymer and an increase in the elongation at break of the material. The peak value of loss factor tanδ increases from 0.405 to 1.213 in tension mode and from 0.412 to 0.825 in compression mode, and the effective damping temperature range is broadened from 82.5 °C to 186.5 °C. This study not only successfully prepares a high-performance IPNs resin functional material but also reveals that precisely tailoring the phase separation morphology of IPNs by adjusting the prepolymer block length is an effective strategy to achieve high-performance damping materials with a wide temperature range. It provides a simple method and idea for the subsequent design and development of high-performance resin-based IPNs polymer materials.