The invention discloses a method for preparing rebaudioside-M by an enzymic method. The method comprises the steps of utilizing tomato UDP-glycosyltransferase and potato sucrose synthetase, and using rebaudioside-A and sucrose as raw materials for producing rebaudioside-M. According to the method disclosed by the invention, the high-yield recombinant strain of UDP-glycosyltransferase and sucrose synthase is obtained by virtue of a genetic engineering technology at first, then a bio-enzyme crude product is collected, a catalytic reaction is directly carried out, UDP or UDP-glucose is not added in the reaction system, the rebaudioside-A and the sucrose are used as the raw materials, and the rebaudioside-A is catalyzed to generate the rebaudioside-M through the efficient circulation of catalyzing the UDP-glucose by the sucrose synthase, and through the action of the tomato UDP-glycosyltransferase. Compared with the existing preparation method for rebaudioside-M, the process is short in steps, low in cost, important in application value, and good in economical efficiency.