The invention discloses a multi-strainliquid straw composting agent, belonging to the technical field of solid waste treatment. The liquid straw composting agent is prepared by the steps of cutting straws of wheat and corns, mixing with rural soil and saccharose, standing, soaking the straws,filtering,collecting filtrate, inoculating the filtrate to a self-made liquid culture medium, carrying out culturing to obtain bacteria liquid, cultivating and activating four bacteria by virtue of a self-made oblique plane culture medium, flushing the culture medium, collecting flushing liquid, inoculating the flushing liquid to a self-made fermentation culture medium, carrying out fermental cultivation, filtering, collecting filtrate, combining the filtrate, mixing the filtrate with the bacteria liquid, finally, concentrating, and collecting a concentrated solution, namely the liquid straw composting agent. Bacterial agents of the liquid straw composting agent are rich, and functional strains are more; and meanwhile, the liquid straw composting agent has relatively good composting effect, the composting cycle is effectively shortened, the growth of next-stubble crops is not influenced, the both production and income of the crops can be promoted, and the liquid straw composting agent has relatively good market promotion values.