TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preparation (Claimed): Preparation of (A) comprises subjecting a UGT gene segment and a PYes carrier to double enzyme digestion of restriction enzymes EcoR I and Sal I, linking a purified product of the UGT gene segment with the PYes carrier to obtain a recombinant plasmid PYes2-UGT, transforming the recombinant plasmid PYes2-UGT to a DH5 alpha competent cell to obtain recombinant Escherichia coli DH5 alpha -PYes2-UGT, selecting the positive clone, and guiding the plasmid of the identified positive clone into Saccharomyces cerevisiae YPH499.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The inducible expression method comprises inoculating the genetic engineering bacteria onto a culture medium taking 20 g/l of glucose as a carbon source according to inoculation amount of 2-5%, culturing the bacteria for 8-16 hours and then collecting the thalluses, transferring the thalluses into a culture medium, taking 20 g/l of galactose as a carbon source for inducing the thalluses for 48-60 hours, where: the culture medium comprises 6.7 g/l of yeast nitrogen base (YNB), 20 g/l of glucose, 0.1 g/l of adenine, 0.1 g/l of arginine, 0.1 g/l of cysteine, 0.1 g/l of leucine, 0.1 g/l of lysine, 0.1 g/l of threonine, 0.1 g/l of tryptophan, 0.05 g/l of aspartate, 0.05 g/l of histidine, 0.05 g/l of isoleucine, 0.05 g/l of methionine, 0.05 g/l of phenylalanine, 0.05 g/l of proline, 0.05 g/l of serine, 0.05 g/l of tyrosine and 0.05 g/l of valine, when glucose is used as carbon source; and the culture medium comprises 6.7 g/l of YNB, 20 g/l of galactose, 0.1 g/l of adenine, 0.1 g/l of arginine, 0.1 g/l of cysteine, 0.1 g/l of leucine, 0.1 g/l of lysine, 0.1 g/l of threonine, 0.1 g/l of tryptophan, 0.05 g/l of aspartate, 0.05 g/l of histidine, 0.05 g/l of isoleucine, 0.05 g/l of methionine, 0.05 g/l of phenylalanine, 0.05 g/l of proline, 0.05 g/l of serine, 0.05 g/l of tyrosine and 0.05 g/l of valine, when the galactose is used as carbon source. The stevioside is transformed into the rebaudioside A by a whole cell catalysis method. The constructing method further comprises taking genetic engineering bacteria subjected to inducible expression as a whole cell catalyst and then adding into a surfactant for changing the permeability of the cell, taking stevioside and glucose as substrates and then adding into magnesium ion and a substance for regulating metabolism, and reacting the substance to obtain the rebaudioside A. An amount of the genetic engineering bacteria is 2 g/l in terms of wet thalluses. An amount of the stevioside is 1 g/l. The surfactant is Pluronic F-68 (RTM: Nonionic polyoxyethylene-polyoxypropylene block co-polymer) (1-10 g/L). The magnesium ion is magnesium chloride (1-10 g/l). The substance for regulating metabolism is uridine monophosphate (0.5-3 g/l), succinic acid (5-10 g/l), orotic acid (1-5 g/l), or citric acid (10-20 g/l). The reaction is performed in a potassium phosphate buffer liquid system at a pH value of 6.8-7.8, and a temperature of 25-42 degrees C for 12-96 hours.