To effectively improve the operation bandwidth and efficiency of power amplifiers, an X-band high-efficiency continuous class B power amplifier was proposed based on 0.25-μm GaN high electron mobility transistor (HEMT) process. The power amplifier adopted the output second harmonic tuned method and utilized the output capacitance of the transistor to design a parallel LC harmonic tuned network, which simplified the circuit structure and optimized the parallel LC harmonic tune network. The second harmonic load impedance and fundamental load impedance were matched accordingly in wideband frequency, satisfying the requirements of continuous class B mode with high efficiency. Furthermore, combined with the second harmonic source-pull method, the power amplifier employed the input second harmonic tuned method and inputted a series LC harmonic tuned network to the output transistor. With the optimization of the series LC harmonic tuned network, the second harmonic source impedance was moved into the high-efficiency regions of the transistor, which achieved the overall improvement of the output efficiency of the power amplifier in the operation bandwidth. Results show that the proposed power amplifier chip was in the bandwidth of 8.0-10.5 GHz with a saturated output power gain of 40.8-42.2 dBm, a saturated output efficiency of 51%-59%, and a power gain of 19.8-21.2 dB. The small signal gain and input return loss of the power amplifier were 23.6-25.6 dB and below-10 dB respectively. The size of the proposed chip was 3.2 mm×2.4 mm. The circuit structure proposed in this paper provides a feasible method to improve the operation bandwidth and efficiency of microwave monolithic integrated circuit (MMIC) power amplifiers.