The invention belongs to the technical field of polymer composite materials, and relates to a glass fiber reinforced nylon composite material and a preparation method thereof. The composite material is composed of the following components in parts by weight: 100 parts of nylon, 35 to 50 parts of fire retardant, 7.5 to 15 parts of charring agent, 0.2 to 1.0 part of coupling agent, 0.1 to 0.5 part of antioxidant, 0.2 to 1.0 part of lubricant, and 400 to 100 parts of glass fiber. A fire retardant namely melamine polyphosphate is added so as to effectively solve the problem of candle wick effect of glass fiber reinforced nylon, and the modified nylon has the characteristics of strong charring performance, no dripping, no toxicity, and no corrosion. Moreover, a charring agent is added, and a synergetic fire retardant effect is generated by the cooperation of the charring agent and the melamine polyphosphate, so the fire-retardant performance of the nylon composite material is greatly improved.