Based on the frequent occurrence of fire accidents on insulation materials rigid polyurethane (RPUF), which can cause great economic loss and casualties, RPUF was prepared by one-step water-blown method and hope to reduce the fire hazards in reality. A small and medium-scale insulation flame spread experimental platform was built, which was used to reveal the flame spread characteristics over RPUF and flame retardant RPUF. The flame structure, flame spread speed, flame temperature and mass loss rate etc. characteristics were analyzed. The results showed that the surface of all the samples was charred during flame spread. The non-flame retardant RPUF showed the largest combustion intensity, while the flame retardant expanded graphite (EG), aluminum hypophosphite (AHP) and aluminum diethylhypophosphite (ADP) resisted the flame spread in some degree, respectively, which was shown in the reduced flame spread speed, mass loss rate, and flame temperature parameters etc. It was also found that RPUF/AHP5 flame spread could sustain a period, while after 20 s the extinguishment was formed. It was because that, the flame retardant RPUF/AHP5 released non-combustion gas when heated, and AHP decomposed into the phosphorus-containing compound, which promoted the formation of charcoal in the polyurethane molecular chain. At the same time, flame retardant RPUF/ADP5 samples distinguished during flame spread, however, the distinguish degree was lower than that of flame retardant RPUF/AHP5. Meanwhile, the surface temperature had two peaks during the flame spread over RPUF/EG5, which were attributed to the instability of the char layer. When the temperature was higher than 400℃, the char layer was oxidized immediately and the heat penetrated the char layer, and unburned material continued to pyrolysis, therefore, the second temperature peak was formed.