Abstract: Waveguide-based power combiner combining microwave beams is an effective approach for enhancing the output capacities of narrow-band high power microwave devices. A specific quasi ? -junction high power microwave combiner is designed for combining the X band microwave beams of the same frequency and the detailed design method and simulation results are presented. Furthermore, the proposed combiner is connected with the coaxial high power microwave source based on dual beams and simultaneously simulated by the Particle-in-cell (PIC) code KARAT. The results indicate that when the diode voltage is 730kV, the guiding magnetic field is 0.8T, the inner beam current is 7.4kA, the outer beam current is 15.6kA, a microwave with power of 4GW and frequency of 9.75GHz is generated after the two microwaves produced by coaxial high power microwave source based on dual beams are combined by the power combiner.