Ammonium phosphomolybdate (AMP) was modified by metal salts, which was compounded with the intumescent flame retardant(IFR) for flame retardant polyoxymethylene (POM).The flame retardant properties and mechanical performance of the POM composites were evaluated, and the flame retardant mechanism was analyzed by cone calorimetry analysis, thermo-gravimetric analysis , scanning electron microscope (SEM),etc.The results show that ammonium phosphomolybdate metal salt (PMos) combined with IFR has excellent flame retardant performance in POM. With 2% PMos and 28% IFR synergistically added in POM, not only the vertical combustion grade(UL94) (3.2 mm) can reach V-0 grade, but also the UL94 (1.6 mm) can reach V-1 grade, and the limit oxygen index(LOI) can reach up to above 58%. Among them, the LOI of AMP modified by magnesium and manganese ion reaches 62.1%. Compared with the flame retardant POM composite without synergist, the heat release rate and total smoke release are decreased by 28.8% and 35.8%, respectively, and the mechanical properties of the composite is also improved.