The invention discloses a microbial compound inoculum for the production of cellulosic ethanol and a preparation method thereof, relating to a compound inoculum and a preparation method thereof, and solving the problems that the cost of fermentation production of cellulosic ethanol is high due to the adoption of a rinsing and detoxification method, and the ultimate yield of the ethanol is low because the rinsing and detoxification method loses the nutrition elements in the lignocellulose. The microbial compound inoculum is mainly prepared from a trichoderma koningii spore suspension, an aspergillus oryzae spore suspension and a chaetomium cupreum spore suspension. The preparation method comprises the following steps: firstly, mixing the trichoderma koningii spore suspension, the aspergillus oryzae spore suspension and the chaetomium cupreum spore suspension; and secondly, mixing and inoculating the mixed spore suspension in a culture medium for cultivation to obtain the microbial compound inoculum. The microbial compound bacterium has few dosage and reduces the cost of the fermentation production of cellulosic ethanol, and the ethanol prepared from the microbial compound inoculum has high ultimate yield.