The invention belongs to the technical field of vegetable straw recovery treatment and relates to a preparation method and application of a complex microbial agent for straw degradation. The method includes steps: S1, collecting vegetable straw, cutting into fragments, and smashing to obtain a straw waste material A; S2, well mixing the straw waste material A, culture soil, a nitrogen source and water to obtain a mixture B; S3, taking the mixture B, performing inoculation of EM bacterial fluid, saccharomycete fluid, bacillus subtilis fluid and trichoderma harzianum fluid, stirring, mixing, andfermenting for 3-5 days to obtain the complex microbial agent. The invention further provides a straw organic fertilizer and a straw returning treatment method. The complex microbial agent for strawdegradation has advantages that antagonism of dominant growth florae can be improved, flora and microbial diversity is improved, and great straw degradation effects and low production cost are realized.