Based on the development of a magnetron injection electron gun(MIG) for gyrotron oscillator operating at 140 GHz and with about a megawatt output power, the temperature homogeneity of the cathode and thermal deformation of the MIG were analyzed with ANSYS code. Under an improved temperature homogeneity of the cathode, the geometrical and electrical parameters have been adjusted and optimized to eliminate the effect of thermal deformation on beam trajectory. The simulated temperature of the cathode will be compared with the tested one for evaluating the rationality of the simulation model, which may be helpful for the actual design.