The invention relates to the technical field of catalyst preparation, and discloses a preparation method and application of a palladium/carbon material catalyst with palladium nanoparticles immobilized by a carbon material in situ. The preparation method includes the following steps that 1, acetic acid is added into water and stirred to prepare an acetic acid aqueous solution; 2, palladium acetateand the carbon material are put into the acetic acid aqueous solution and uniformly stirred to prepare a mixed solution; 3, the mixed solution prepared in step 2 is put into an ultrasonic cleaning machine for ultrasonic dispersion, and then the mixed solution is taken out and then stirred; 4, the mixed dispersion liquid prepared in step 3 is placed into a rotary evaporator and dried to prepare the palladium/carbon material catalyst with the palladium nanoparticles immobilized by the carbon material in situ. The method has the advantages of being small in energy consumption, low in cost, environmentally friendly, easy to use on a large scale and the like. The prepared palladium/carbon catalyst is applied to a reaction of preparing benzaldehyde through selective oxidation with benzyl alcohol, the rate of converting benzyl alcohol into benzaldehyde can reach 96-99.3%, and the selectivity can reach 98-100%.