Abstract: Although the added phosphorus flame retardant can improve the flame retardancy of polyurethane foam, its poor compatibility with the matrix and uneven dispersion will degrade the application performance of polyurethane foam, and further restrict its application in building insulation, cold chain logistics and other fields. Using red phosphorus(RP) as the core material and isophorone diisocyanate(IPDI) as the shell material, a red phosphorus microcapsule (MRP) flame retardant was prepared by interface polymerization, and was used to improve the foaming performance, thermal insulation and flame retardant performance of polyurethane composite foam (PCF). The morphology and structure of MRPs were characterized by scanning electron microscopy, X-ray energy spectrum, infrared spectrum, infrared thermal imaging and other means, and the influence of the addition of MRPs on the foaming performance, microstructure, combustion insulation and flame retardant performance of PCF was studied. The results show that the synthesized MRP has a core-shell structure, a uniform morphology and size, and an average particle size of 9.09 μm. When 1.5wt% MRP was introduced into PCF, not only did the excellent foaming and thermal insulation properties of PCF were retained to the maximum extent, but also achieved a significant improvement in PCF flame retardant performance. The flame retardant grade can reach UL-94 V0 grade and the LOI value reached 26.3%. This work provides a new method for the preparation of phosphorus based flame retardants and provides a reference for the application of phosphorus based flame retardants in thermal insulation materials.