The invention discloses a long glass fiber reinforced flame-retardant ABS composite material and a preparation method thereof. The composite material comprises polycarbonate, ABS, a flame retardant material, modified fibers, modified graphene oxide and other components, and the surface of the modified graphene oxide is treated with a silane coupling agent, so that the compatibility of the grapheneoxide and matrix components (polycarbonate and ABS) is improved, the graphene oxide and the matrix components can be uniformly dispersed in a system, the modified fiber is mainly prepared from blackphosphorene, dopamine, absolute ethyl alcohol and pretreated fiber, the pretreated fibers are glass fibers with carbon nanowires growing on the surfaces, and the glass fibers are continuous glass fibers. By utilizing the synergistic effect of the aluminum hydroxide, the magnesium hydroxide, the polysiloxane, the modified graphene oxide and the black phosphorene, the ABS composite material has excellent flame retardance, and is low in smoke, low in toxicity and higher in practicability during combustion.