Nanoalloys, especially high-entropy nanoalloys (HENAs) that contain equal stoichiometric metallic elements in each nanoparticle, are widely used in vast applications. Currently, the synthesis of HENAs is challenged by slow reaction kinetics that leads to phase segregation, sophisticated pretreatment of precursors, and inert conditions that preclude scalable fabrication of HENAs. Here, we report direct conversion of metal salts to ultrafine HENAs on carbonaceous support by nanosecond pulsed laser under atmospheric conditions. Because of the unique laser-induced thermionic emission and etch on carbon, the reduced metal elements were gathered to ultrafine HENAs and stabilized by defective carbon support. This scalable, facile, and low-cost method overcomes the immiscible issue and can produce various HENAs uniformly with a size of 1 to 3 nanometers and metal elements up to 11 with productivity up to 7 grams per hour. One of the senary HENAs exhibited excellent catalytic performance in oxygen reduction reaction, manifesting great potential in practical applications.