The invention relates to a synthesis method of a thiophanate side chain, which comprises the following steps: using a material shown in the description as a raw material, using ketone reductase to reduce to produce a product shown in the description, and using monooxygenase to oxidize to produce a product shown in the description, after protection of para-tosyl, carrying out sulfonyl protection toobtain a target product shown in the description, according to the technical scheme, by using the ketone reductase and the monooxygenase, a first step of biotransformation and a second step of biotransformation are carried out respectively, the oxidation product in the second step is subjected to Ts (para-tosyl) protection, finally, sulfonyl protection is carried out to obtain the target product,because the first step and the second step are both biotransformation, extraction by using ethyl acetate is not needed, the use amount of the ethyl acetate is reduced.