The invention discloses a composite for waterproof breathable film and a preparation method of the composite. The composite for the waterproof breathable film is prepared from 29-49 parts of metallocene polyethylene, 3-8 parts of high pressure polyethylene, 45-55 parts of inorganic fillers, 3-8 parts of reinforcing agents, 0.1-1 part of antioxidant, 0.5-1 part of lubricant and 0.2-1 part of coupling agents by mixing, extruding and granulating. The reinforcing agent added can effectively improve melt strength of the composite and the composite is not easy to be broken when being pulled, so thatproduction efficiency of cast film can be increased greatly, particularly, the production rate can be up to 150-180 meter / minute, and compared with the conventional production rate of 80-120 meter / minute, the production efficiency is increased by 50%. The preparation method of the composite is simple and low in price, and capable of effectively reducing production cost of the cast film.