In order to improve the flame retardant properties of polyurethane foams, aramid fiber (AF) modified by phosphoric acid was added to the rigid polyurethane foams. Oxygen index, smoke density meter, cone calorimeter, and thermogravimetric analyzer were used to characterize the flame retardancy, fire behavior, thermal stability and mechanical properties of the rigid polyurethane foams and flame-retardant rigid polyurethane foams. The results show that the polyurethane foams containing modified AF (MAF) have better flame retardant, smoke suppression and mechanical properties than the polyurethane foams containing AF. Compared with the pure polyurethane foams, the LOI for the polyurethane foams containing 5% content of MAF is enhanced by 15.8%, max smoke density, peak heat release rate, total heat release, peak smoke production rate and total smoke production release values are decreased by 25%, 25.3%, 10%, 35.7%, 47.3% respectively. MAF can improve the thermal stability of the polyurethane foams, and the residual mass at 700 ℃ is 14.5%, which is more than that of pure polyurethane foam.