The present invention discloses a Se-doped less-layer WS2 nano-sheet/nitrogen and phosphorus co-doped graphene composite nanometer material preparation method, which comprises: S1, dispersing graphene oxide in water, adding a quaternary phosphonium salt, uniformly mixing, sequentially adding ammonium thiotungstate, selenium powder and hydrazine, and uniformly mixing; and S2, carrying out a hydrothermal reaction on the mixed dispersion system obtained in the step S1, cooling, collecting the solid product through centrifugation, washing, drying, and carrying out a heat treatment in a nitrogen or argon atmosphere so as to prepare the Se-doped less-layer WS2 nano-sheet/nitrogen and phosphorus co-doped graphene composite nanometer material. According to the present invention, graphene oxide, ammonium thiotungstate, selenium powder and hydrazine are adopted as the raw materials, and the quaternary phosphonium salt assisted hydrothermal method and the subsequent heat treatment method are used to successfully prepare the Se-doped less-layer WS2 nano-sheet/nitrogen and phosphorus co-doped graphene composite nanometer material; and the method has advantages of simpleness, convenience, and easy industrial application expanding.