Flame-retardant PA66 composites were prepared by melt blending with bisphenol Abenzoxazine (BOZ) as a carbon synergist anddiethyl-phosphate aluminum (ADP). Their synergistic flame retardant effects and mechanism were investigated by means of UL94 vertical burning rating, limiting oxygen index(LOI), cone calorimeter test（Cone）, thermogravimetricanalysis(TGA) and scanningelectronmicroscope(SEM).The results showed that BOZ and ADP had good synergistic flame retardancy in PA66. Introducing proper amount of BOZ can not only improve the flame retardancy of the PA66 composites, but also significantly enhance the thermal stability especially the initial decomposition temperature. Moreover the mechanical properties of the composites are not much worse. When 0.3wt%BOZ and 7.7wt%ADP were added, the ADP/BOZ/PA66 composites reached UL94 V-0, LOI arrived to 32.8%, and their tensile strength and bending strength were 81.52MPa and 111.11MPa respectively. The study of flame retardant mechanism showed that both ADP/BOZ and ADP are dominated by gas phase flame-retardant mechanism and accompanying simultaneously condensed phase flame-retardant mechanism.