AbstractA microencapsulated flame retardant PMDOPO was synthesized by solution blending method using EGDMA as initiator and AIBN as cross‐linking agent. Rigid polyurethane foams were prepared in a one‐step process. PMDOPO was incorporated into the foams by blending with polycarbonate diols (PCDL) and foaming additives. The results of Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) showed that PMDOPO was well formed and uniformly dispersed in the foam. The pore size and density of the foams first increased and then decreased with the addition of PMDOPO. X‐ray photoelectron spectroscopy (XPS), vertical combustion test, and thermogravimetric analysis (TGA) showed that the PMDOPO content was directly proportional to the flame retardancy and the thermal stability. The major weight loss shifted to higher temperature intervals. The amount of residual char of RPUF/PMDOPO‐25 was enhanced 50 times to 10.63% compared to pure RPUF. In addition, SEM and Raman analyses of the combustion residual char revealed that a phosphorus‐containing char layer was formed during thermal degradation. The dense charcoal layer helps to insulate heat and suppress combustion. The universal testing machine showed that RPUF with 15 wt% content had the best compression resistance, with an enhancement of 1.8 MPa over pure RPUF.