A microwave power module (MPM), which is a hybrid combination of a solid-state power amplifier (SSPA) as a driver and a traveling-wave tube amplifier (TWT) as the final high power stage, is a high-power device largely used for radar applications. A gain equalizer is often required to flatten the TWT output power gain owing to its big gain fluctuations over the operating frequency range. In this paper, the design of an X-band, fully-coaxial, easily-tunable broadband power equalizer for an MPM is presented. The structure is composed of a coaxial waveguide as the main transmission line and a coaxial cavity loaded with absorbing material as a resonant unit. Sensitivity analyses of the attenuation amplitude and resonant frequency of the equalizer in terms of coaxial cavity length, thickness of the absorbing disc, and insertion depth of the probe were carried out. The measured results were in good agreement with the simulated ones, showing that the equalization curve met the requirements well and proved that this optimal structure has the advantages of a large power capacity, a wide operating frequency band, is easily tunable, and good transmission performance.