Abstract: The amplification of continuous-wave backward signal in 1 μm high-power master-oscillator-power-amplifier based Yb-doped fiber laser was investigated using rate equation model. The results show that the backward light power would be amplified significantly by the high-power amplifier. The 100 W backward signal from the output end of the fiber amplifier can be amplified to up to kW level. Meanwhile, the amplification of backward signal can consume the population inversion, saturate laser gain and thus decrease the laser output power seriously. Furthermore, the backward signal amplification would result in a much higher laser intensity at the incident end of the amplifier gain fiber, where the highest pump power existed. The temperature at the incident end of the fiber can be 100 ℃ higher than that without backward signal. The higher laser intensity at the incident end could break the pump absorption saturation, and enhance the rates of pump absorption and stimulated emission a lot, hence increased the thermal load and the temperature significantly. Since the backward signal gain was determined by the saturation of population inversion by the forward seed, the power fluctuation of oscillator caused by the amplified backward signal may aggravate the backward signal amplification, and further increase the risk of damage. Higher forward seed power resulted in stronger saturation of the laser gain in the active fiber, which could suppress the backward signal amplification effectively. However, higher seed power put forwards much complex requirements to laser oscillator, and the thermal load in the active fiber of the laser amplifier would be more concentrated, which made the thermal management more difficult. Furthermore, with higher seed power, the stimulated Raman scattering and thermal induced transverse mode instability are more likely to occur. Therefore, it is important to optimize the seed laser power based on a comprehensive consideration of the above issues, and to prevent the backward light from coupling into the fiber amplifier.