The invention belongs to the technical field of genetic engineering, and particularly relates to novel high-efficiency gamma-glutamyl methylamine synthetase and a plasmid-free genetic engineering bacterium for L-theanine production and construction and application thereof. The plasmid-free genetic engineering bacterium which performs denovo synthesis on L-theanine efficiently by taking cheap carbon sources such as glucose as a substrate is provided, escherichia coli serves as a host, and gamma-glutamyl methylamine synthase genes gmas-Mu copied three times are integrated on a genome of the escherichia coli; a glutamate dehydrogenase gene Cgl2079 is copied once; a pyruvate carboxylase gene Cgl0689 is copied once; a citrate synthase gene gltA is copied once, and the genetic engineering bacterium is obtained. After metabolic transformation of a system, the engineering bacterium can perform denovo synthesis on the L-theanine by taking the glucose as the raw material, the fermentation yieldand sugar-acid conversion rate are the highest values reported so far, in fermentation of a 5 L fermentor, the maximum production of the L-theanine can reach 60 g/L, and the sugar-acid conversion ratecan reach 40%.