Red-phosphorus masterbatch and magnesium hydroxide were used to prepare halogen-free flame retardant synergist HIPS composites.The mechanical properties were studied by universal testing machine,and the flame retardant effects were studied by means of horizontal-vertical burning and LOI.The results indicated that the mechanical properties and the flame retardant properties of composites were affected by the ratio fraction of RPM and MH when the concentration of synergistic flame retardant was the same.