The invention discloses a method for preparing trifluoromethylbenzaldehyde. P-trifluoromethyl chlorobenzene, nitromethane and triethylamine are adopted as initial raw materials, an intermediate 1-nitro methyl-4-benzotrifluoride is prepared through a substitution reaction without a solvent or under the condition that a solvent exists, and then the target product trifluoromethylbenzaldehyde is prepared through an oxidizing reaction under the action of ozone. The mass percentage content of obtained trifluoromethylbenzaldehyde ranges from 98.1% to 99.5% (liquid chromatogram, external standard method), and the total yield ranges from 90.3% to 96.5% (by p-trifluoromethyl chlorobenzene). The reaction equation is shown in the specification. According to the method, the synthetic raw materials and the process routine are improved and innovated, no metal catalyst is adopted any more, the raw materials are cheap and easy to obtain, the process procedure and operation are easy, aftertreatment is easy, a high-quality p-benzaldehyde product can be prepared through two popular reaction steps, namely, the substitution reaction and the ozonation reaction, and the reaction yield and product quality are both high.