The invention discloses a method for preparing levofloxacin carboxylic acid by hydrolyzing levofloxacin cyclization ester. The traditional method has larger corrosion on equipment, poorer safety in the production process and poor environmental protection in an operation filed. The levofloxacin carboxylic acid is prepared by the following steps of: forming a hydrolyzed mixed solution by phosphoric acid, C2-C4 organic monacid and water, adding the levofloxacin cyclization ester into the hydrolyzed mixed solution and carrying out a hydrolyzation reaction at 85-125 DEG C; and then carrying out temperature reduction, crystallization, filtration, rinsing and drying to obtain the levofloxacin carboxylic acid. By the method, not only the levofloxacin cyclization ester can be completely hydrolyzed, but also the prepared levofloxacin carboxylic acid has favorable color and luster and high content; in addition, the technical defects and relevant problems of two traditional processes for hydrolyzing levofloxacin cyclization ester can be solved.