Abstract: The flame retardantcy polyethylene (PE) matrix composites were prepared by twin-screw extruder, with hydromagnesite (HM) and Mg(OH)₂ as the flame retardants. The flame retardant properties and mechanism of HM-Mg(OH)₂/PE flame retardant composites were investigated by limited oxygen index (LOI), vertical buring test (UL-94), cone calorimetric test (CCT) and universal tensile test. The thermal decomposition behavior of HM-Mg(OH)₂/PE flame retardant composites was studied by thermogravimetric analysis (TGA). The results show that, the flame retardant in appropriate proportions of HM and Mg(OH)₂ decomposes in the combustion temperature range which leads to a better flame retardantcy. In the premise of the same LOI and tensile strength, the cost of PE composite with synergistic flame retardant decreases significantly compared with Mg(OH)₂ as flame retardant alone. The ineffective even negative decomposition of HM reduces in the synergistic flame retardant, and the inhibitory effect on the decomposition by the PE matrix with HM under high temperature is retained, at the same time, a relatively stable scaly protective layer forms in the combustion region surface, in adition, the total lost mass of synergistic flame composite is higher. With those variety of factors, the flame retardancy effect of the synergistic flame composite is higher. The HM-Mg(OH₂)/PE flame retardant composites pass the UL-94 V-0 rating with LOI value of 28%, and the tensile strength reaches 28.8 MPa when the mass ratio of HM to Mg(OH)₂ is 1:2 with 60wt% loading.