This letter proposes a novel design method for resistive-resistive series of continuous modes (Res-Res SCMs) power amplifier (PA), which can extend the bandwidth of the SCMs PAs. Using a waveform engineering method, by introducing resistive part to the purely reactive second harmonic load of the series of continuous modes, overcoming the bandwidth limitation of traditional continuous PAs within one octave due to the inability to overlap between low band and high-order harmonics and high band fundamental waves. An ultra-wideband PA working across 0.4–3.1 GHz (154.3% bandwidth) is simulated and fabricated using a 10 W GaN HEMT device, the experimental results show that a drain efficiency of 63%−81.7% can be achieved with a saturated output power of 38.6−42.3 dBm in the desired design band.