Abstract: Three types of phosphates, dimethyl phosphate (DP), (5-ethyl-2-methyl-1,3,2-dioxaphosphorinan-5-yl) methyl dimethyl phosphonate P-oxide (EMD) and dimethyl methylphosphonate (DMMP) were used as typical add-type flame retardants for the flame retardancy of rigid polyurethane foam (RPUF), and expandable graphite (EG) was also added as a flame-retardant additive. The flame-retardant behaviors and mechanisms of the resulting composite foams were investigated. The results indicated that the composite foams containing DMMP and DP exhibited greater limiting oxygen indices than that containing EMD at the same content of phosphate flame retardant. The composite foam containing DMMP also presented the lowest heat release and total heat release rates. Owing to the oxidation reaction of DP during combustion, the heat release rate increased, resulting in a reduction in flameretardant effect. Although EMD could be decomposed into DMMP molecules during combustion, its flame retardant efficiency was lower than that of DMMP due to a low content of phosphorus. DMMP exhibited the best flame-retardant effect due to its high phosphorus content and high flame retardant efficiency in both gas and condensed phases.