An extended interaction oscillator(EIO) based on a single cavity composed of 12 periods of staggered double rectangular waveguide slow wave structure (SDRWSWS) is modeled and calculated on computer, those methods and steps for determining the structural parameters and the beam parameters are given. A new method called “phase re-synchronization technology” is proposed to improve electron efficiency of EIO, the method is to decrease the periods of the fifth and sixth periods of SDRWSWS counted from the entrance of the beam by 10%, so that the magnitude of the z component of electric field intensity becomes weaker in the segment which is far from the output port, but becomes stronger in the segment which is near from the output port, and such an E_z distribution is in favor of good bunching in the beam as the beam is proceeding. On the other hand, the phase of the z component of electric field intensity increases about 51.6o between the 7 to 12 periods, so that the electric field maintains synchronization with the space electric charge wave in the beam and abstracts more kinetic energy from the beam. Computer simulation results show that both power and electron efficiency become higher conspicuously, the maximum improved values are over double of unimproved ones.