The radical suppression of the photodarkening effect and laser performance deterioration via H2 loading were demonstrated in high-power Yb-doped fiber (YDF) amplifiers. The photodarkening loss at equilibrium was 114.4 dB/m at 702 nm in the pristine fiber, while it vanished in the H2-loaded fiber. To obtain a deeper understanding of the impact of photodarkening on laser properties, the evolution of the mode instability threshold and output power in fiber amplifiers was investigated. After pumping for 300 min, the mode instability threshold of the pristine fiber dropped from 770 to 612 W, and the periodic fluctuation of the output power became intense, finally reaching 100 W. To address the detrimental effects originating from photodarkening, H2 loading was applied in contrast experiments. The output power remained stable, and no sign of mode instability was observed in the H2-loaded fiber. Moreover, the transmittance at 638 nm confirmed the absence of the photodarkening effect. The results pave the way for the further development of high-power fiber lasers.