Abstract: Using magnesium hydroxide (MH), aluminum trihydrate (ATH) and microencapsulated red phosphorus (MRP) as halogen-free flame retardants and high impact polystyrene (HIPS) as polymer matrix, a series of MH-ATH-MRP/HIPS composites were prepared by melt-compounding the flame retardants and HIPS. The flame retardancy of the composites was investigated by means of horizontal burning, vertical burning, oxygen index, cone calorimetry and high temperature pyrolysis experiments. It is shown that the composite containing both MH and ATH exhibits better flame retardancy than its counterpart containing only MH or ATH at the same flame retardant contents. When the mass ratio of MH-ATH/HIPS is 70:30:100, the horizontal burning rating of the obtained composite can reach FH-1 and its oxygen index is 25.2%, but its vertical burning cannot meet any rating. The introduction of a tiny amount of MRP, whose mass fraction is 2.9% of the composite, to the above composite can make the fire performance index of the composite increase by 85% and slow the heat release and mass loss upon combustion. Meanwhile, the charring ability of the composite is enhanced considerably and the vertical burning reaches FV-0 rating. The composite displays optimal fire retardancy when the mass ratio of MH-ATH-MRP/HIPS is 21:9:12:100, thus reducing the loading of flame retardant remarkably. There is very prominent synergistic flame retardant effect among MH, ATH and MRP to HIPS. The coexistence of MH and ATH can not only suppress the temperature rise and pyrolysis of HIPS in a much wider temperature range, it can also release water vapor one after another in a much wider temperature range, which dilutes the concentrations of both oxygen and combustible gases. Thus, they have a synergistic effect in HIPS composites. The charring ability of the composite is enhanced appreciably after the introduction of MRP, which has further improved the effect of condensed phase flame retardation. Consequently, the fire retardancy of the composites is increased considerably.