The invention provides a composting method for biologically dehydrating livestock manure, which comprises the following steps: under the condition of adding biological fermentation strains, horizontally laying the livestock manure, adding fly maggots for fermenting, and then, adding the livestock manure and biological fermentation strains in batches, wherein the fermentation period is 3-7 days, and the fermentation condition is 22-42 DEG C. The invention also provides a composting method for biologically dehydrating the livestock manure, which comprises the following steps: under the condition of adding the biological fermentation strains, horizontally laying the livestock manure, and then, adding the fly maggots for fermenting, wherein the fermentation period is 7-12 days, and the fermentation condition is 10-22 DEG C. The invention reduces the initial water content of livestock manure composting by microbiological synergistic action, fully performs the action of composting fermentation strains, does not need to add auxiliary materials for regulating the water content of the livestock manure, reduces the treatment cost of the livestock manure, and improves the composting quality. Meanwhile, the culture of fly maggots provides a large amount of high-quality feed protein and raw materials for further processing to better recycle organic wastes, thereby realizing the purposes of protecting environment and increasing efficiency and yield agriculturally.