A cathode-adjustable single anode magnetic control electron gun belongs to a single anode magnetic control electron gun that is used in conjunction with a vacuum gyrotron, and comprises an electron gun casing, a base, an electric heating wire component, a support cylinder, a regulating mechanism assembly, an electron gun cathode, a fixed annular plate, an anode and a retainer ring, wherein, the electric heating wire component comprises an electric heating wire, an electric heating wire sleeve and insulating fillers, and the electric heating wire sleeve is provided with a ring-shaped scale section, a threaded section and a guiding plane surface; the regulating mechanism assembly is arranged between the electric heating wire sleeve and the support cylinder thereof, and comprises an adjusting screw nut; in addition, the cathode-adjustable single anode magnetic control electron gun can adjust the distance between the cathode and anode according to the operating condition, the transverse to longitudinal velocity ratio of an obtained electron beam is greater than 1.0, and the longitudinal velocity spread is less than 2 percent. Therefore, the influence caused by assembly error and overheating deformation or great current and voltage variation of the electron gun can be overcome effectively, and the cathode-adjustable single anode magnetic control electron gun is characterized in that working current and voltage capable of adapting to wider range can be achieved, the discreteness of the electronic longitudinal velocity is low, the performance of the electron beam is excellent, and the power and the efficiency for later beam-wave interaction of the gyrotron can be improved and so on.