Polyoxometalate is a new type of catalyst, which has good application in many fields. The properties of phosphomolybdate (PMo) can be regulated by changing the metal cations. In this work, phosphomolybdic acid (PMA) anions were reacted with three different metal ions (Ni, Na and Zn) to form PMos, which were used as catalysts to improve the flame retardant efficiency of polypropylene/intumescent flame retardant (PP/IFR). The results show that the PP composites can obtain the UL-94 V0 grade of flame retardant efficiency by adding 25wt% IFR. However, if PMo is introduced into PP/IFR system, only 14.5wt% IFR and 0.5wt% sodium phosphomolybdate (NaPMo) or sodium phosphomolybdate (ZnPMo) are needed for PP to achieve the UL-94 V0 grade. While under the same formulation, nickel phosphomolybdate (NiPMo) can only make PP composite obtain the UL-94 V1 grade. Different metal ions have different catalytic activities in PP/IFR, among which NaPMo and ZnPMo match with IFR better than NiPMo. PMos can promote the reactions among IFR, slow down the heat release rate during combustion, and form a char layer with better barrier effect, so as to improve the matching of PP and IFR, and improve the flame retardant efficiency in the UL-94 test.