High energy radiation such as γ ray induces a darkening effect on Yb-doped fiber (YDF), adversely affecting the reliability of Yb-doped fiber laser (YDFL). In this study, we assess the influence of the total dose of γ ray on the loss of large mode area YDF. The results show that the radiation induced loss (RIL) of the tested fiber linearly increases with the total dose. The optical efficiency of YDFL with different structural parameters after the generation of RIL is then investigated based on the measured value of RIL and rate equation model. The amplifier scheme pumped with 976-nm laser diodes exhibits the least sensitivity to the total dose of γ ray. The optimized central wavelength used herein is 1070 nm for the YDF; however, the efficiency difference is only 2% for wavelengths from 1060 nm to 1100 nm. This paper performs the research based on the YDFL system, providing a reference for further YDFL design and applications.