The energy crisis has led to the development of building materials in the direction of high quality. Rigid polyurethane foam (RPUF) is an insulation material with many excellent performances. However, the rigid polyurethane foam has the defect of being extremely easy to be ignited, and there is a potential safety hazard in the actual use, so it needs to be treated with flame retardant. Piperazine pyrophosphate (PAPP) is a flame retardant that contains both phosphorus and nitrogen flame retardant elements, and it can be added to the RPUF system which can solve the problem of being weak in flame retardancy of RPUF. The limiting oxygen index (LOI), thermogravimetric (TG), and cone calorimetry (CCT) have been used to investigate the flame retardant properties and combustion performance of PPAP/RPUF composites. Scanning electron microscopy (SEM) demonstrated the microscopic vesicle structure of RPUF, and the results showed that the addition of PAPP increased the inhomogeneity and breakage of the vesicle structure. The flame retardant test indicated that the addition of PAPP could effectively inhibit the dripping of the composite, with the highest LOI value of the composite material at 22.7vol% and passing the UL-94 test at V-0 level with the addition of 50wt% PAPP. TG test conducted that the decomposition of PAPP was divided into three stages, and the decomposition rate of its second stage was the largest. The acid substances produced by the decomposition of PPAP made RPUF matrix have a higher char residue rate, in which the char residue of PAPP50/RPUF increased to 34.4wt% at 700℃, and the char residue plays a role in blocking heat and releasing combustible gas in the condensed phase. CCT test confirmed that PAPP could inhibit the release of the heat of combustion from the composite material, and the peak heat release rate (PHRR) and total heat release (THR) generated by PAPP50/RPUF combustion were 54% and 41% lower than those of pure sample. The mechanical properties test demonstrated that the addition of PAPP increased the thermal conductivity and reduced the compressive strength of the PPAP/RPUF composites.