The invention provides a complex bacteria inoculant for preparing an organic fertilizer from effluents of animal slaughter houses and breeding plants serving as raw materials, which has the advantages of effectively deodorizing and enhancing the fertility, and a preparation method thereof. The complex bacteria liquid inoculant is prepared by the following steps of: independently performing liquidfermentation on each single bacterium, namely A, lactobacillus fermentium, B, bacillus laterosporus, C, trichoderma koningii, D, nitrogen-fixing bacteria spores and E, bacillus mucilaginosus; and mixing the bacteria, wherein the final product comprises the living bacteria (including living spores) in percentage by weight: 20 to 50 percent of A, 20 to 50 percent of B, 0 to 20 percent of C, 0 to 20percent of D and 0 to 20 percent E. The liquid inoculant is further processed so as to obtain solid inoculant. The complex inoculant of the invention is specially used for preparing the organic fertilizer from excrements of pig farms (or chicken farms) and wastes of slaughter houses thereof, which serve as basic raw materials, and a small amount of straws, and has an obvious effect of reducing odor of the fertilizer and the field. The complex inoculant combines the functions of improving the disease resistance of plants, promoting growth and improving soil and the fertility of the fertilizer.