In the attempt to promote microwave technology to terahertz band, the key problems are frequency, efficiency or power of the device. Based on the Maxwell equations, the beam dynamics and beam-wave conversion efficiency were discussed for the microwave oscillator with two-cavity structure. In the framework of classical mechanics, the motion equation of electronic phase was reduced to the pendulum equation. And from the phase plane properties of the system, the acceptance and the beam-wave conversion efficiency of the device were discussed. The results show that when the phase space density is in uniform distribution, larger phase area results in stronger beam and higher power; and the greater the energy spread is, the stronger is the interaction of beam-wave and the higher is the conversion efficiency.