The invention discloses a porous platinum/palladium alloy nanoparticle catalyst as well as a preparation method and application thereof. The preparation method comprises the following steps: mixing hexadecyl trimethyl ammonium chloride aqueous solution, chloroplatinic acid aqueous solution and sodium borohydride aqueous solution to obtain seed solution, then injecting the seed solution into growth solution prepared from hexadecyl trimethyl ammonium chloride aqueous solution, chloroplatinic acid aqueous solution, chloropalladic acid aqueous solution and ascorbic acid aqueous solution, growing for a period of time, so that nano-porous dendrite platinum palladium alloy nanoparticles with palladium enriched at the center are successfully obtained, then oxidizing for stripping the palladium at the center, so that the porous hollow cherry-shaped platinum/palladium alloy nanoparticle catalyst is obtained. Compared with the prior art, the preparation method is simple, reaction conditions are mild, the preparation method is simple and practicable and is green and pollution-free; the nano catalyst adjustable in dimension and components is obtained through controllable synthesis, and the obtained platinum/palladium alloy nanoparticle catalyst has larger specific surface area and shows excellent electrochemical catalytic performance.