Abstract: The flame retardant modification of nylon 66 (PA66) was carried out by melt blending method using aluminium diethyl phosphate (ADP) and halloysite nanotubes (HNTs) as flame retardant system. The effects of the ratio of ADP to HNTs on the flame retardant properties, mechanical properties and thermal stability of ADP-HNTs/PA66 composites were studied. The combustion carbon layer was observed by SEM. The flame retardant mechanism was studied by TG-IR and FTIR. The results show that the flame retardancy of the ADP-HNTs/PA66 composites increases first and then decreases with the increase of the proportion of HNTs. When the flame retardant system is 11wt%ADP-1wt%HNTs, the UL94 flame retardant grade of the ADP-HNTs/PA66 composite is V-0 grade, the limiting oxygen index (LOI) is 35.6%, and it has synergistic flame retardant effect. The tensile strength and elongation at break increase with the increase of the proportion of HNTs in the ADP-HNTs flame retardant system, while the impact strength decreases gradually. TG analysis shows that HNTs can promote charring and slow down the degradation. SEM shows that ADP-HNTs flame retardant system can form a continuous dense carbon layer. TG-IR and FTIR analysis show that ADP has both gas phase and condensed phase flame retardancy, and the introduction of HNTs can interact with ADP in the condensed phase to promote cross-linking and charring.