The invention discloses a method for preparing benzaldehyde by using cyclodextrin polymer to catalyze the oxidation of cinnamic aldehyde or cinnamon oil. The method comprises the following steps: taking the cinnamic aldehyde or the cinnamon oil as raw materials, adopting the cyclodextrin polymer as a catalyst, taking one of hydrogen peroxide, sodium hypochlorite or tert-butyl hydroperoxide as an oxidizing agent, taking water as a solvent, controlling the reaction temperature to be 30-90 DEG C, after finishing the reaction, using an ester solvent, methyl alcohol, acetone or acetonitrile to conduct extraction, and obtaining the benzaldehyde by decompression and concentration. The method has the advantages of simple technique, mild reaction condition, low cost, high yield, high natural levelof the benzaldehyde, environment friendliness, easily-separable catalyst with reusability and the like.