Abstract: A design of a broadband high-efficiency power amplifier based on extended continuous class-B/J mode is described in this paper. A bandwidth-enhanced dual-path impedance matching technology is proposed that extends the continuous Class-B/J mode to achieve resistive second harmonic impedance as the design goal, and combines it with a bandwidth-extended dual-branch tuning network. By simultaneously manipulating four frequency points, the best second harmonic impedance matching is achieved in a wide frequency band, thereby achieving bandwidth expansion and efficiency improvement of the power amplifier. In order to validate the proposed design theory, a multi-octave prototype is designed and fabricated, and the design idea is verified by using a 10 W gallium nitride high electron mobility transistor device. The implemented power amplifier has a relative bandwidth of 140% in the 0.45 GHz ~ 2.55 GHz bandwidth range. In addition, an output power of 39.3 dBm ~ 42.4 dBm, a drain efficiency of 61.2% ~ 76.7%, and a gain of 9.3 dB ~ 12.4 dB can be achieved within a wide bandwidth.