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This invention relates to a method for synthesizing 1-cyclopropyl-4-oxo-7-fluoro-8-methoxy-1,4-dihydroquinoline-3-carboxylic acid, a key intermediate for numonoxacin. The method uses 2,4-difluoro-3-methoxyacetophenone as a starting material and proceeds through four steps: haloform reaction, acyl chloride formation and Maxwell's acid condensation, reaction with dimethylformamide dimethyl acetal and cyclopropylamine, and base-catalyzed cyclization and acid deprotection. This efficient method yields the target intermediate. The synthetic route of this invention utilizes readily available raw materials, mild reaction conditions, and simple operation. The multi-step reaction can be carried out in a one-pot process, achieving a total conversion rate of approximately 85% and a product purity exceeding 99.5%, making it suitable for industrial production.