The invention provides a synthesis method for gemcitabine. The method takes the aldol condensation reaction with acetonide condensed glyceraldehydes as a key reaction which is induced by a chirality prosthetic group. The method comprises the following steps: protecting the newly generated hydroxyl; carrying out the acidic hydrolysis of the acetonide; selectively protecting a primary hydroxyl; obtaining aldehyde through reduction and prosthetic group abscission; simultaneously realizing cyclization, hydroxyl sulfonylation, condensation, and hydrolysis protection; and finally obtaining gemcitabine after crystallization.