The invention discloses a preparation method of a flux suitable for traveling wave tube cathode brazing. The preparation method comprises the following steps: (SS1) tungsten powder is put in a hydrogen furnace for annealing once at 850-900 DEG C; and the temperature is kept by 0.5-1 hour; (SS2) cobalt oxide powder is put in the hydrogen furnace for reduction at 800-850 DEG C before material preparation; the temperature is kept by 15-20 minutes; and pure cobalt powder is prepared; (SS3) the tungsten powder and the cobalt powder heated in the steps (SS1, SS2) are milled, and are screened by screens of 200-600 meshes; (SS4) the tungsten powder and the cobalt powder are respectively weighed according to a weight ratio of 22.22: 77.78 to put in a corundum mortar for mixing and grinding until the tungsten powder and the cobalt powder are totally mixed to prepare a sample mixture; and the grinding time is 50-60 minutes; and (SS5) the flux melting point inspection is performed for a certain quantity of the sample mixture prepared in the step (SS5); and when the melting point range of the sample fixture is within 1450-1500 DEG C, the batching is qualified. The preparation method solves the problem that a cathode cannot crack and fall during working at high temperature.