Abstract: The disclosure discloses a method to elevate the amount of steviol glycosides remained in the recrystallization mother liquor of Stevia extract, and belongs to the field of modification and extraction of natural compounds. The disclosure adopts lactase for highly specific catalytic hydrolysis of stevioside to convert stevioside into rubusoside which usually can only be obtained from leaves of Rubus suavissimus S. Lee; and meanwhile, after stevioside in the mother liquor glycosides is converted, since the polarity difference between rubusoside and rebaudioside A is far larger than the polarity difference between stevioside and rebaudioside A, rebaudioside A is more easily separated out of the mother liquor glycosides. A technology provided by the disclosure is also applicable to value increment of mixed steviol glycosides containing stevioside and is beneficial for separating out rebaudioside A from the mixed steviol glycosides.