The invention discloses a flame-retardant polyurethane composite material comprising components including, by weight, 20-40 parts of sucrose polyether, 50-70 parts of isocyanate, 10-15 parts of nano-grade alumina powder, 2-4 parts of a quaternary ammonium salt catalyst, 6-9 parts of a flame retardant, 3-5 parts of a compatibilizing agent, 0.5-1 part of an antioxidant, and 2-2.5 parts of a dispersing agent. The composite material provided by the invention has high flame-retarding performance. The oxygen index is no lower than 30. The smoke density is low, and is only 70% of that of a common flame-retardant polyurethane rigid foam product. A fuming rate is also substantially reduced. Fire-resistant and thermal insulation performance of the material is greatly improved. With good performance, the material has good application prospects in aspects such as thermal insulation pipes, composite plates, fireproof blockage materials, fire door core materials, partition wall core materials, and the like.