The present invention provides a method for preparing bean dregs soluble dietary fiber by zymotechnics. Firstly, to dry the fresh bean dregs at the temperature of 60 to 80 DEG C for 12 to 14 hours, then to comminute until passing 40 mesh sieve, and to confect into ferment culture medium according to the ratio of 1 : 20 to 25 of bean dregs : water, and regulating the pH value to 4 to 4.5, then to disinfect at the temperature of 121 DEG C for 15 to 20 min to obtain liquid; to inoculate leeb wood mildew into the culture medium according to the 3 to 4 percent of the liquid volume ratio, then to shake bed culture at the speed of 140 to 160 r/min for 3 to 4d at the temperature of 28 to 30 DEG C. After sterilization, filtering and concentrating, to obtain deposit by non-water ethanol deposition, then to dry the deposit in vacuum to produce finished product. The present invention condition is geniality, equipment is simple, enzyme specificity is high, and can improve activity component amount maximum, has no pollution to circumstance, the SDF content in the bean dregs is increased greatly.