The invention relates to compound microbial decomposed inoculas, and a preparation method and an application thereof. Effective components of the involved compound microbial decomposing inocula comprise bacillus amyloliquefaciens Ba168, paenibacillus peoriae BC-39, trichoderma harzianum, candida krusei, aspergillus niger and streptomyces griseus. The preparation method comprises that each strain are fermented separately, and the strains are mixed for use according to the proportion after the completion of the fermentation. The involved application is an application of the compound microbial decomposing inocula in degradation of crop straw. The compound microbial decomposing inocula not only give full play to unique degradation function of the bacillus on the straw, are combined with microbial compound flora for efficient synergy in the straw degradation process, thereby efficiently and rapidly degrading the straw, and significantly enhancing soil organic matter and humus levels.