The invention discloses a composite microbial inoculum for composting organic materials and a preparation method thereof. The microbial inoculum mainly comprises pichia pastoris, lactobacillus acidophilus, bacillus coagulans, enterococcus faecalis, bacillus natto, bacillus subtilis, aspergillus nidulans, white-rot fungi, aspergillus usamii, trichoderma harzianum, trichoderma reesei and aspergillus niger. The preparation method comprises the following steps: first classifying the microorganisms into bacteria, yeasts and filamentous fungi, then performing mixed culture on the same class of microorganisms till quality requirements are met, and then compounding according to the following proportions: 20-70 parts of bacteria, 30-60 parts of yeasts and 50-80 parts of filamentous fungi. The compositing microbial inoculum prepared by the preparation method has the advantages of fast fermentation start during composting fermentation, high temperature in the composting process, a long high-temperature period and fast organic material decomposition. An organic fertilizer prepared by using the microbial inoculum is good in quality; compost has a fertility-retaining function, has high effective utilization rate of a large number of elements such as nitrogen, phosphorus and potassium, and has the effects of dissolving phosphorus, dissolving potassium and fixing nitrogen; compared with a control group which is not inoculated with the microbial inoculum, a product provided by the invention has the advantages as follows: the content of water-soluble organic carbon is increased by about 2 times, the total humic acid content is increased by about 1 times or above, and the content of biochemical fulvic acid is increased by about 2 times.