By power ultrasonic, nano silicon dioxide particles were dispersed into polyaryl-polymethylene-isocryanate(PAPI)and rigid polyurethane foams reinforced by nano SiO₂ were prepared by polymerization PAPI with polyether polyol. SEM was employed to observe the structures of SiO₂ filled polyurethane foam and showed that nano SiO₂ particles had been dispersed uniformly in foam. Results of mechanic test show that nano SiO₂ particles in low loading fraction can increase the compressed strength and impact strength of the rigid polyurethane foam. But the addition of nano SiO₂ conduces to the rapid accretion of viscosity of PAPI, which makes the foam reaction difficult to set in and the impact strength decreases with the addition of 7% nano SiO₂.