The invention provides a method for preparing an alloy nanoparticle. The method utilizes good SPR properties and local heating ability of precious metal materials and heat insulation and original shape maintaining functions of SiO2 nano layers, so that different metal atoms in a multi-metal structure of a core-shell structure migrate to form the multi-metal alloy nanoparticle with constant morphology. The method uses sunlight or simulated sunlight as a light source without the need for other energy sources, the method is green, environmentally friendly and simple and quick to operate, completely alloyed nanoparticles of various morphologies can be prepared, and the prepared alloy nanoparticle has good dispersion performance, uniform size and excellent stability.