The invention relates to a preparing method of a composting fermentation strain. Preparing and using methods comprise the following steps: collecting microorganisms in an environment; inoculating a selective medium, which takes a rice leachate, sugar, milk, inorganic salt, bran, rice husks, composting raw material chippings as major ingredients, with the microorganisms, and implementing directional screening and propagation, so that the compound strain which is applicable to composting is obtained; forming a trapezoid material pile by virtue of such composting raw materials as crop straws, vegetable leavings, excrements of livestock, kitchen waste and the like; regulating the trapezoid material pile to a proper water content; uniformly inoculating the trapezoid material pile with the composting fermentation strain; turning the pile for several times and implementing aerobic fermentation until composting raw materials become soft, the volume of the composting raw materials is diminished, the morphology of the raw materials cannot be observed obviously, and the composting raw materials are neutral or faintly alkaline and free from odor, so that composting is completed. With the application of the methods, composting time can be obviously shortened, the thorough decomposition of the composting raw materials can be accelerated and a more thorough fermentation reaction can be guaranteed.