The invention discloses a one-pot method used for preparing guacetisal under visible light catalysis. According to the one-pot method used for preparing guacetisal under visible light catalysis, 2-(2-methoxy phenoxyl)benzoic acid is taken an initial raw material, a photosensitizer is added, salicylic acid o-methoxy phenyl ester is obtained under visible light irradiation, no post-treatment is needed, triethylamine and acetic anhydride are added into a reaction system directly so as to realize one-pot preparation of guacetisal. In the preparation process, visible light is used, energy consumption is reduced, one-pot preparation is adopted, post-treatment is simple, operation is simple, and total yield is high.