Abstract: Provided is a method for preparing Rebaudioside M by using an enzyme method. In the method, Rebaudioside A or Rebaudioside D is used as a substrate, and in the presence of sucrose and UDP, Rebaudioside M is generated by means of reaction of the substrate under the catalysis of a mixture of UDP-glucosyl transferase and sucrose synthetase, or recombinant cells containing the UDP-glucosyl transferase and sucrose synthetase. The reaction is carried out in an aqueous-phase system at a temperature of 20 to 60° C. and pH 5.0 to 9.0. The reaction system further contains dimethyl sulfoxide at a concentration of 3% to 5% according to the ratio by volume for facilitating solubilization of the substrate.