The invention discloses a lithium battery case flame-retardant material based on long-glass-fiber-reinforced polypropylene and a preparation method thereof. The lithium battery case flame-retardant material is prepared from the following components in parts by weight: polypropylene, continuous glass fiber, coupling agent, main antioxidant, auxiliary antioxidant, lubricant, compatilizer, composite flame retardant and black master batch. The preparation method comprises the following steps: adding all the components expect the continuous glass fiber into a high-speed mixer, uniformly mixing, sending into an extruding machine, carrying out melt extrusion on the material into an impregnator, and meanwhile, sending the continuous glass fiber into the impregnator to carry out disperse impregnation with the molten material; and finally, carrying out die drawing at a setting port of the impregnator, cooling, carrying out blow-dry, and granulating to obtain the finished product. The lithium battery case flame-retardant material disclosed by the invention has higher strength and toughness, can not be easily combusted in naked fire, can effectively lower the possibility of battery combustion accidents, and ensures the case to have high plasticity and compressive strength.