The flame retardancy properties of magnesium hydroxide (MH) in benzoxazine were investigated by cone calorimeter and UL-94 vertical burning test. The results indicate that, in UL-94 test, burning time shortens and flame retardancy of polybenzoxazine is improved after the addition of magnesium hydroxide. When the amount of MH is equal to or greater than 20% in the benzoxazine resin, V-0 rating can be achieved. The cone calorimeter test reveals that, with the addition of 20% MH, time to ignition (TTI) increases from 33s to 52s, peak heat release rate (PHRR) decreases from 635.4 kW/m2 to 255.93 kW/m2, total heat release (THR) reduced from 99.1 MJ/m2 to 67.72 MJ/m2, total smoke release (TSR) decreases from 25785 m2/m2 to 1686.19 m2/m2. For benzoxazine, magnesium hydroxide is a kind of environment friendly flame retardant and smoke suppressant.