The new type DOPO-MMT nanocomposite was added to the polyurethane elastomer to obtain a novel polyurethane elastomer composite. Its flame retardancy and flame retardant mechanism were studied. And its flame retardant effect was compared with separately-added DOPO, montmorillonite(MMT) and the physical mixture of DOPO and MMT. The results show that the flame retardant effect of DOPO-MMT flame retardant is better than that of DOPO, MMT and physically mixed DOPO and MMT flame retardants, because DOPO-MMT achieves nanometer level in polyurethane.The gas phase flame retardation of DOPO and the coagulation phase flame retardation of MMT is complementary to each other, and the flame is suppressed, so that the UL-94 vertical combustion of the polyurethane composite material reaches the V-2 level, and the lowest heat release measured by the cone calorimetry test. The TPU/DOPO-MMT composite obtains a good flame retardant effect.