The invention discloses a composite microbial inoculant for sludge treatment and a preparation method thereof. The composite microbial inoculant is prepared from Escherichia coli, Bacillus and Aspergillus niger in a weight ratio of (1-5):(0-2):(0-3). The composite microbial inoculant can be mixed with a sludge mixture to perform composting, and the sludge can be fermented, heated and ripened within a time period, so that the harmful microbes and pathogens in the sludge can be effectively killed by enhancing the temperature; and thus, most water is evaporated, and heavy metals are converted from the soluble state into the insoluble state, thereby obtaining the organic fertilizer which satisfies the requirements for agricultural use.