This paper derived time domain multimode formula and combined it with the frequency single mode theory to investigate mode oscillation and mode interaction of a 394 GHz gyrotron. The gyrotron reaches a steady state where both and modes oscillate at 393.87 GHz in time domain theory while cannot oscillate in frequency domain theory when the beam voltage is 15 kV, the magnetic field is 7.185 T, and beam current is 0.25A. mode dominates the final oscillation, which output power and efficiency are 136.8 W and 3.6%, respectively. The output characteristics of the operating mode obtained by two theories are identical.