An encapsulated structure of an X ray generator with a cold cathode and method of vacuuming the same are disclosed. The X ray generator has a glass ball-tube having a base, a tungsten filament, a cold cathode, a focus cap, and an anode target inside, associated with a first electrode pin, a second electrode pin, a single-used pin, and anode pin extended out. The tungsten filament located at the periphery of the base has a first wire end connected with the second electrode pin and a second wire end connected with the single-used pin. While vacuuming the glass ball-tube before melting an end to seal, a voltage is exerting on the single use pin to heat the tungsten, and a high voltage is exerting on the anode target to accelerate the hot electrons emitting from the filament to bombard the inside wall of the glass ball-tube and the anode target so as to shorten the vacuuming time and increase the vacuum level.