The invention discloses an environment-friendly composite gel breaking agent and a preparation method and application thereof. The environment-friendly composite gel breaking agent is composed of, by weight, 100 parts of distilled water, 10-20 parts of cation strong electrolytes, 10-30 parts of carbon nanomaterials and 0.3-1.0 part of octadecyl trimethyl ammonium chloride. The method comprises the steps that firstly, the 10-30 parts of carbon nanomaterials, the 10-20 parts of cation strong electrolytes, the 0.3-1.0 part of the octadecyl trimethyl ammonium chloride and the 100 parts of distilled water by weight are sequentially added into a beaker, and the mixture is subjected to ultrasonic operation for 10-30 minutes; then the mixture is moved into a stainless steel reaction kettle, and the kettle is capped, sealed and placed in an oven to be heated for 12-36 hours at the temperature of 150-200 DEG C; and the kettle is taken out and naturally cooled to be at the room temperature. The invention provides application of the environment-friendly composite gel breaking agent to processing of waste drilling fluid. The environment-friendly composite gel breaking agent has the beneficial effects that the moisture content of mud cakes is 27.9%-30.7%, the chemical oxygen consumption (CODCr) of filtrate is 70-98 mg/L, and chroma of chroma filtrate is 50-80.