The development of materials which meet the needs of both cost-efficiency and high performance for hydrogen evolution reaction is of great importance. However, developing photocatalysts with Pt-like activity still remains as a major challenge. Herein, we utilize ultrafine amorphous ruthenium phosphide (RP) nanoparticles as a high efficient and robust cocatalyst to enhance the H-2 production activity of g-C3N4 (g-CN). The RP/g-CN samples were prepared based on a facile in-situ growth phosphatization method. The optimum Hy evolution rate reached up to 2110 mu mol h(-1)g(-1) for 0.1%-RP/g-CN, which was 113.4 times as high as that of pristine g-C3N4 and 2.22 times of Pt-loaded g-C3N4. Furthermore, ruthenium is the cheapest platinum-group metal and its amount in the best RP/g-CN sample is only 0.1%, showing the superiority of competitive price and high activity. The introduction of ultrafine amorphous ruthenium phosphide accelerated the transfer rate of electrons and restrain the recombination of charge carriers. The amorphous ruthenium phosphide ultrafine nanoparticles could also serve as cocatalysts for hydrogen evolution. This work provides a promising alternative to expensive Pt-loaded photocatalyst for excellent hydrogen evolution performance under visible light irradiation.