The invention discloses a green process for preparing diosgenin and belongs to the technical field of natural product extraction. The main procedure of the process comprises the steps of washing, smashing, naturally fermenting, emulsifying, separating fibers, separating starch, separating emulsion, performing acidolysis, separating oil and water, performing reduced-pressure condensing, and recrystallizing to get the product; recovering acid and monosaccharide in the acidolysis solution through a simulated moving bed; and adding a nonpolar solvent into a natural fermentation solution while grinding by a colloid mill, emulsifying the diosgenin to form the emulsion fixed in the water, and separating to get the cellulose, starch, supernate and emulsion after emulsifying. The emulsion is directly acidolysis processed so that the hydrolysis condition is reduced, the diosgenin is free of being damaged, and the quality of the diosgenin is increased; the pollution source is completely eliminated by the acid and the monosaccharide in the acidolysis solution. The process meets the standards of high purity, high yield, low cost and pollution free. The prepared diosgenin content is greater than 98%, the yield is greater than 80%, the recovery rate of the cellulose, the starch and the like is greater than 95%, the production cost is reduced by 30%, the ton cost is reduced by **-** and no pollution is discharged.