Urethane modified bismaleimide (UBMI) was synthesized by the reaction of maleic anhydride (MA) with NCO group terminated polyurethane prepolymer (PUP) in presence of acetone. The product was determined by infrared analysis. Then ultrasonic assistant process was introduced into the solvent removal of the prepolymer mixture of UBMI and PUP. Polyurethane-imide (PUI) elastomer was synthesized from the above PUP-UBMI prepolymer mixture by the infusion technology with 2,5-dimethyl-2,5-bis(tert-butylperoxy)-hexane (B25) as liquid initiator at 120°C. The thermal properties and stress-strain behavior of PUI elastomer was characterised by thermogravimetric (TG) analysis and tensile testing apparatus, respectively. Compared with pure polyurethane elastomer, the PUI elastomer composite showed the better thermal stability.