To achieve high power, short-pulse CO₂ laser outputs, a 13.56 MHz radio frequency (RF)-excited axial fast-flow CO₂ laser amplifier with an adjustable RF injection power of 0?88 kW is developed in this study. Additionally, a laser-amplification experimental device is created to investigate the gain performance of the amplifier. First, the six-temperature model theory is described and the laser-amplification kinetic equation is established and its output characteristics are calculated. Second, the relationship between amplifier RF injection power, CO₂ proportion, non-dissociation ratio, and other parameters and the small-signal gain coefficient under three different cavity pressures is analyzed. When the amplifier cavity pressure is 8 kPa, the RF injection power is 50 kW, and the CO₂ ratio is 14%, the maximum small-signal gain coefficient is obtained. As the RF injection power continues to increase, the small-signal gain coefficient first increases and then gradually saturates. Reasons contributing to the amplifier-gain saturation are analyzed theoretically. Based on experimental measurements, when the seed light input power is 110 W, the amplifier output power can exceed 3500 W. Finally, the evolution of the laser-pulse waveform during the gain-extraction stage is simulated and the time-domain variation characteristics of the small-signal gain coefficient are analyzed.