The invention relates to a method for promoting to biosynthesize 1,3-propylene glycol by adding reducing sugar, which comprises the following steps: preparing a fermentation medium used for the 1,3-propylene glycol; by taking glycerol as a fermentation substrate, inoculating a Klebsiella pneumonia strain culture solution into the fermentation medium to carry out anaerobic fermentation or microaerophillic fermentation to synthesize the 1,3-propylene glycol; and in the fermentation process, introducing the reducing sugar of which the fermentation system concentration is 1-50g / L. In the method for promoting to biosynthesize the 1,3-propylene glycol by adding the reducing sugar, according to the characteristic of xylose, mannose, rhamnose and the like producing a large number of reducing equivalents in the metabolic pathway in a bacterium body, by utilizing the principle that the biosynthesis of the 1,3-propylene glycol can be promoted because of the existence of more reducing equivalents, a good amount of reducing sugar is added to the fermentation medium to increase the reducing equivalents, and thereby, the synthesis concentration and the conversion rate of the 1,3-propylene glycolare improved. The method for promoting to biosynthesize the 1,3-propylene glycol by adding the reducing sugar is simple to operate, is capable of improving the bioavailability and reducing the fermentation cost and is suitable for industrial production.