Abstract: Particular simulations on the 34GHz Gyrotrons magnetic injection gun(MIG) consider the influences of such design parameters as: control anode voltage, beam current, structure of MIG and external dc magnetic field. The results show that when control anode voltage increases or magnetic induction intension near cathode decreases, beam velocity ratio will increase while beam velocity spread decreases; as known, beam motion in the gun is affected by the interaction of magnetic field and space charge force, when beam current changes, the minimal velocity spread will gain at a best current; when the magnetic field compression ratio changes lower, the best current value will raise, and beam velocity ratio decrease, meanwhile minimal velocity spread will be a new higher value. The structure of the