The invention relates to a construction method of escherichia-coli gene engineering bacterium generating L-carnitine and an application. The gene engineering bacterium is characterized by converting encoded crotonbetaine into three key genes of the L-carnitine, namely caiB, caiC and caiD, transferring the three key genes, a transporter encoding gene caiT and a positive regulator caiF into escherichia coli for overexpression, controlling by an anaerobic promoter or adopting IPTG to induce expression of related genes for synthesis of the L-carnitine, and simultaneously deleting aceK gene for encoding isocitrate dehydrogenase phosphatase / kinase of the escherichia coli. After the gene enginering bacterium is cultured by production enzyme, and thalli are collected as a whole-cell enzyme source; the crotonbetaine is directly converted to generate the L-carnitine, after conversion, the yield of the L-carnitine can reach more than 30g / L, the molar conversion rate is 42% at highest, the yield and the conversion rate are increased by more than 60 times than those of wild plants, and the construction method reaches the leading level of biological-process preparation reported domestically. The gene engineering bacterium has a high conversion rate for substrate, the enzyme-reaction process is simple, the cost of production materials is low, the resource is saved and no pollution is caused.