The invention discloses a recombinant bacterium. The recombinant bacterium is used for expressing 2-ketoisovalerate deacidification 
enzyme (also called 
ketoacid decarboxylase). The recombinant bacterium is obtained by modifying a 
genome of a recipient bacterium, wherein the modification comprises the step of enabling the recipient bacterium to express the 2-ketoisovalerate deacidification 
enzyme, and the recipient bacterium is 
pseudomonas or 
escherichia coli. The method for synthesizing the propionic acid by using the recombinant bacterium is characterized in that the propionic acid is synthesized from 2-ketobutyric acid under the 
catalysis of 2-ketoisovalerate deacidilase. A novel 
metabolic pathway for synthesis of propionic acid is provided. The transformation rate of propionic acid is 96.67% when the recombinant 
pseudomonas PS32 obtained after transformation in the invention is used for catalyzing 
threonine synthesis for 24 hours; and after material supplementation, the conversion rate of the propionic acid can be 98.47% during 
catalysis of 
threonine synthesis by the PS32. The 
recombinant escherichia coli obtained after transformation in the invention can be used for transforming L-
threonine with a concentration of 400 mM into 
propionate with a concentration of 393 mM within 24 hours, and a conversion rate is 98.25%.