The invention relates to a preparation method of nanometer WC-Co composite powder, which belongs to the technical field of preparation of a nanometer composite material. The preparation method is characterized in that nanometer WC powder of which the grain size is 30-200 nm is used as a raw material, the raw material of the nanometer WC powder can be subjected to low-temperature ultrasonic chemical plating by adding plating solution in the nanometer WC powder after the nanometer WC powder is sensitized, cleaned, activated, cleaned and dispersed by adding a dispersing agent under an ultrasound condition, the plating liquid for chemical plating is formed by 14-26 g/l of CoSO4.H2O, 100-150 g/l of KNaC4H4O6.4H2O, 18-40 g/l of NaH2PO2.H2O and 40-60 g/l of (NH4)2SO4, and pH (Potential Of Hydrogen) of the plating liquid is controlled to be 8-11 while plating is carried out. According to the preparation method disclosed by the invention, the problem that nanometer WC is easy to unite during a chemical plating process is solved, a uniform metallic cobalt clad layer is obtained on the surface of the nanometer WC, the technological process is simple, the cost is lower, the size of the prepared nanometer composite material is less than or equal to 260 nm, and the defect of the prepared nanometer composite material in application can be greatly improved.