The invention provides a gray Pd/TiO2 nanowire photocatalyst. The gray Pd/TiO2 nanowire photocatalyst is prepared through the following method: adding TiO2 powder into an NaOH aqueous solution, carrying out uniform mixing under stirring, then carrying out heating to 150 to 200 DEG C, carrying out a hydrothermal reaction for 36 to 48 h, then carrying out filtering, subjecting an obtained filter cake to acid washing until a pH value of the filter cake is neutral, and carrying out washing and drying so as to obtain powdery H2Ti3O7; adding the powdery H2Ti3O7 into an aqueous solution of a Pd precursor, carrying out uniform mixing under stirring so as to obtain a mixture, subjecting the mixture to drying at 60 to 80 DEG C for 10 to 15 h, then placing the dried mixture in a flowing hydrogen atmosphere, carrying out heating to 150 to 600 DEG C, and carrying out calcination for 3 to 5 h so as to obtain a product. According to the invention, a gray Pd/TiO2 nanowire is prepared by utilizing Pd surface hydrogen overflowing reduction for the first time; the preparation method has the characteristics of low cost, simple operation, greenness and environmental protection; and the catalyst prepared by using the method provided by the invention has the characteristics of high activity, high selectivity and easy recycling in a reaction of preparing benzaldehyde through photocatalytic benzaldehyde solvent-free liquid-phase oxidation.