The invention discloses a method for preparing o-trifluoromethyl benzaldehyde. The method for preparing the o-trifluoromethyl benzaldehyde, disclosed by the invention, comprises the following steps: in the presence of a catalyst, carrying out hydrolysis reaction on a mixture of o-trifluoromethyl methylbenzene bichloride, o-trifluoromethyl chloro-fluoro-methylbenzene and o-trifluoromethyl methylbenzene difluoride and water at the temperature of 80-150 DEG C to obtain, wherein the mass of the catalyst accounts for 0.01-10% of the mass of the mixture. The method disclosed by the invention is cheap and easily available in raw materials, low in cost, little in wastewater, low in energy consumption and simple in operation and can be suitable for industrial production. The structural formula of the o-trifluoromethyl benzaldehyde is shown in the specification.