The invention discloses a direct heating cathode for a high power magnetron and a preparation method thereof. The preparation method comprises the following steps: metal tungsten powder and thorium oxide powder are mixed and ground to obtain tungsten-thorium alloy powder, the tungsten-thorium alloy powder is mixed with a 1.5-3wt% nitrocellulose solution to obtain a mixed solution, the mixed solution is sprayed on the surface of a tungsten wire cathode substrate to be baked under an infrared lamp and a tungsten wire cathode with the tungsten-thorium alloy powder on the surface is obtained, thenthe tungsten wire cathode is put in a high temperature hydrogen furnace to be sintered and high temperature decontamination and carbonization are performed so that the direct heating cathode for thehigh power magnetron is prepared. According to the direct heating cathode for the high power magnetron, the emission current density of the pure tungsten wire cathode for the magnetron can be improved, the working temperature and the surface evaporation rate of the pure tungsten wire cathode can be reduced and thus the service life of the cathode and the magnetron can be prolonged.