For achieving sufficient flame retardancy, high magnesim hydroxide (MH) content is needed in MH flame retardant Acrylonitrile-butadiene-styrene copolymer (ABS) composite (ABS/MH), which will cause difficulty in processing and poor mechanical property. We prepared modified flame retardant MH, fumed silica (SiO2) and zinc borate (ZB) via TX-10 phosphate/polyacrylate latex as well as MH flame retardant ABS with the modified flame retardant of 60.0wt% and studied the synergistic flame retardant effect of SiO2 and ZB with MH in ABS/MH. The incorporation of ZB, SiO2 or SiO2/ZB could prolong the time to ignition, reduce the average and peak heat release rate of ABS/MH in cone calorimeter test.