The invention relates to a high stability composite guaiacol phenolsulfonic acid oral liquid and a preparation method thereof, and belongs to the technical field of pharmaceutics. The oral liquid is prepared by adopting promethazine hydrochloride, guaiacol phenolsulfonic acid, ammonium chloride, sodium citrate, benzoic acid, ethylparaben, cane sugar, caramel, Litchi chinensis essence and banana essence as raw and auxiliary materials, adding an antioxidant cysteine hydrochloride or vitamin C and adopting ethanol and water as a solvent. According to the invention, deficiencies of conventional formula are improved, and the antioxidant is added to effectively prevent promethazine hydrochloride form being oxidized. The guaiacol phenolsulfonic acid oral liquid prepared by the formula and process is high in stability in the placing process, so that the medicine quality and medication safety of patients can be effectively ensured.