The invention discloses a method for preparing WC-Co nano composite powder by in situ reduction and carbonization, which comprises the following steps: 1, preparing a mixed solution containing W, Co and C or mixed solution containing W, Co, C and inhibitor raw materials or an emulsion; 2, carrying out hydrothermal reaction; arranging the mixed solution obtained in step 1 in a hydrothermal reaction kettle, sealing, heating to 120-200 DEG C, preserving the temperature for 2-15h, cooling to the room temperature, and then filtering, cleaning and drying the reactants to obtain presoma powder; 3, reducing and carbonizing; and 4, directly arranging the presoma powder obtained in step 2 in a vacuum furnace or protective atmosphere furnace, and heating to 800-1100 DEG C to carry out reduction and carbonization reaction with the reaction time of 0.5-6h to obtain WC-Co and WC-Co nano composite powder containing an inhibitor. The raw material used in the method is cheap, the process is simple, the product is pure, particles are fine and uniform, the defects of long cycle, nonuniform mixing and the like of a ball-milling mixed material process can be avoided, the preparation cycle is shortened, the production cost is reduced, and the method has the advantage of industrial application.