Developing effective, stable and cheap hydrogen evolution reaction (HER) cocatalysts have attracted great interest. Herein, noble-metal-free cocatalyst CoP/Co3O4 composite is synthesized via a simple low temperature (300 degrees C) solid phosphidation reaction using cobalt hydroxide and sodium hypophosphite. The composition, structure, morphology and photoabsorption performance of the as-prepared samples are characterized by XPS, XRD, TEM, UV-Vis DRS and PL technologies. As a HER cocatalyst, the photocatalytic hydrogen evolution activity of CoP/Co3O4 using EY dye as a sensitizer and trimethylamine (TMA) as an electron donor is investigated under visible light irradiation (lambda >= 420 nm). The result shows that CoP/Co3O4 composite exhibits excellent photocatalytic H-2 evolution activity and good stability. The highest apparent quantum yield (AQY) reaches 40.5% under the optimal conditions. The presence of Co3O4 decorated on CoP can promote H2O dissociation in the alkaline medium and decrease the HER overpotential, therefore, the activity of CoP/Co3O4 composite for the H-2 evolution is higher than CoP alone.