The invention discloses an enzyme-bacterium duplex straw decomposing agent and its preparation and application methods. The decomposing agent is made by mixing a composite microbial agent and a composite enzyme agent in a weight ratio of 3-4:1. The composite microbial agent includes Trichoderma viride, Aspergillus niger, Aspergillus oryzae, Brevibacillus laterosporus and a fused Saccharomyces cerevisiae, having a weight ratio of (1-2):(1-2):(1-2):(2-3):(2-3). The composite enzyme agent includes cellulase produced by the fermentation of Trichoderma reesei, hemicellulase produced by the fermentation of Bacillus subtilis, and ligninase produced by the fermentation of Pleurotus ostreatus, having a weight ratio of (1-2):(1-2):(1-2). The decomposing agent has a double straw degrading function, allowing more efficient and more thorough biological degradation, increasing the straw degrading speed to a higher extent and shortening degrading time, and can fully decompose crop straw to fields within about a week, without affecting the planting and transplanting of next-stubble crops.