Abstract: The rigid polyurethane foam with expansive soil, organic phosphorus, graphite three flame retardant, comparative experiments were conducted on various single and three kinds of flame retardant synergistic flame retardant properties and mechanical properties of rigid polyurethane foam, the experimental results show that with 10 g polyether polyols as a benchmark, when the flame retardants dosage was 60% and the ratio of expansive soil:organic phosphorus:graphite=1:1:2, flame retardant effect is best, the compression strength is 0.129 8 MPa, and oxygen index reached 32, the flame grade is 5VA, all indexes meet the application requirements.