The invention relates to the technical field of biological agricultural chemicals, in particular to a decomposing agent for organic materials and a preparation method thereof. The decomposing agent is prepared from seed solution and a fermentation medium for co-tank fermentation. The seed solution comprises, by weight, 2-5% of acetobacter pasteurianus, 8-12% of bacillus subtilis, 8-12% of bacillus licheniformis, 3-7% of bacillus amyloliquefaciens, 8-12% of bacillus natto, 13-17% of thermophilic sporotrichum, 3-7% of photosynthetic bacteria, 4-8% of thermophilic nocardia, 3-7% of trichoderma koningii, 2-5% of saccharomyces cerevisiae, 5-8% of lactobacillus plantarum, 3-7% of rhizopus formosensis, 2-5% of rice koji, 1-3% of aspergillus niger and 8-12% of bacillus megaterium. The process includes steps of early strain culture, co-tank fermentation and solid powder preparation, and the decomposing agent contains various types of bacterial agents in synergistic effect and is effective in material degradation. The preparation method is simple in procedure, convenient to operate, easy to implement and low in production cost, bacterial antagonism is avoided, and excellent decomposing effects and high practicality are achieved.