Abstract: The present disclosure discloses a quinoline-2,3-fused nine-membered ring scaffold compound, and a preparation method and application thereof as an effective component in a plant fungicide. The quinoline-2,3-fused nine-membered ring scaffold compound stated in the present disclosure is prepared by the following method including steps: mixing a quinoline-derived aniline compound and a formaldehyde compound, adding a solvent and a catalyst, controlling a system temperature, and reacting with stirring to obtain a quinoline-2,3-fused nine-membered ring compound. According to the present disclosure, a 1,6-hydride transfer/cyclization strategy is triggered by aldimine condensation, the quinoline-derived aniline compound and the formaldehyde compound are prepared into the quinoline-2,3-fused nine-membered ring compound with a wide substrate scope and a potential biological activity by a “one-pot synthesis method” quickly.