Disclosed is a process for preparing 5-aminosalicylic acid by gas phase catalytic carboxylation, characterized in that carbon dioxide is introduced into a system of p-acetaminophenol and a basic compound at a temperature of 150° C.˜220° C. and a pressure of 0.5˜5.0 MPa in the presence of a catalyst, so as to carry out a gas phase catalytic carboxylation reaction to form a 5-aminosalicylate, and the crude product 5-aminosalicylate is separated, and then acidified to prepare 5-aminosalicylic acid (5-ASA). Since the gas phase catalytic reaction replaces a solid phase thermo-chemical reaction, the reaction is significantly improved in terms of the process flow, reaction conditions, product quality and energy consumption. In addition, the reaction has a short reaction time, good selectivity, high yield and no formation of wastes during the reaction, and is suitable for industrial production.
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