The invention relates to the technical field of agricultural cultivation, in particular to a method for producing a flemingia philippinensis floating seedling substrate by using bagasse. The substrateis prepared from, by volume, 3-5 parts of bagasse fermentation product, 0.5-1.5 parts of vermiculite and 0.5-1.5 parts of river sand. The method comprises the following steps that (1) the bagasse isspread to be dried for dehydration until the water content is 50-60%, natural stack for fermentation is carried out for 10-12 months; (2) after full decomposition is completed, 110-DEG C vapor is adopted to perform disinfection for 30 minutes, the bagasse is spread to be dried for dehydration until the water content is 10-30%; (3) the bagasse fermentation product after spread drying is pulverizedby a pulverizer and sieved through a 40-mesh sieve; and (4) the pulverized and sieved bagasse fermentation product, the vermiculite and the river sand are mixed uniformly according to a volume ratio of 3-5:0.5-1.5:0.5-1.5 to prepare the flemingia philippinensis floating seedling substrate. The method for producing the flemingia philippinensis floating seedling substrate by using the bagasse has the advantages that the efficient utilization of agricultural waste resources is achieved, not only can grass carbon exploitation be reduced thereby protecting the ecological environment, but also the waste can be changed into treasure, the waste emission and garbage pollution are reduced, and the substrate can return to the field to serve as an organic fertilizer to be used.