This invention relates to an antibacterial compound, more specifically, 6-fluoro-1-methyl-4-oxo-7-(1-piperazine)-4H-[1,3]thiazine[3,2-a]quinoline-3-carboxylic aspartate (compound I), its preparation method and its application. Compound I is prepared by reaction of 6-fluoro-1-methyl-4-oxo-7-(1-piperazine)-4H-[1,3]thiazine[3,2-a]quinoline-3-carboxylic acid and polyaspartic acid. Compound I can be used as effective component and mixed with normal pharmaceutical carrier to manufacture antibacterial drug composition, which can be tablets, capsules, granules, injection, eye preparations, ear preparations, gynecological preparations, and external use preparations. Compound I has stable properties, and can be easily dissolved in water, thus can be easily manufactured into various preparations used in clinical treatment. The method has such advantages as simple and reasonable process, and low cost.