The invention discloses a method for improving the alpha-ketoisovaleric acid yield of Klebsiella pneumoniae, and a modified strain. The method comprises: inactivating acetolactate decarboxylase in Klebsiella pneumoniae, or simultaneously inactivating indole-3-pyruvate decarboxylase and/or lactate dehydrogenase, wherein the Klebsiella pneumoniae inactived with the enzyme is the modified strain; andinoculating the modified Klebsiella pneumoniae in a carbon source culture medium, and carrying out fermentation culture, wherein the carbon source in the culture medium is converted into alpha-ketoisovaleric acid by the modified Klebsiella pneumoniae during the fermentation culture. According to the present invention, with the method, the exogenous gene is not introduced into the modified strain,such that the strain has high genetic stability, the final concentration of the product is high, and the range of the raw material is wide.