Abstract: Prolonging the pulse duration is an important method to increasing the average output power of the high power microwave (HPM) pulses. However, the pulse duration of the HPM is narrow owing to the "pulse shortening" phenomenon. Relativistic backward-wave oscillators (RBWOs) are one of most promising HPM sources for its high output power, high power conversion efficiency, and high repetition operations. It is difficult to prolong the pulse duration while enhancing the beam-wave efficiency. To solve the above-mentioned problem, a long-pulse RBWO with dual resonant cavities is proposed. Firstly, the dual resonant cavities is used to decrease the electric field. Then the non-uniform SWSs are introduced in the device to enhance the beam-wave efficiency. Finally, a large-radius collector is employed to decrease the electron stimulated desorption and secondary electron emission. It is shown in the experimental results that the pulse duration and the beam-wave efficiency are enhanced, compared with the conventio-nal long-pulse RBWOs.