Continuous-wave fiber amplifier plays an important role in the field of manufacture and military. In order to investigate the critical factors that modify the extraction efficiency of continuous-wave ytterbium-doped fiber amplifier, this paper describes a model of a fiber amplifier incorporating amplified spontaneous emission (ASE) based on the steady-state rate equations. The relationships between extraction efficiency and the reflectivity of fiber ends, signal power, pump power are researched. The model by comparing predictions with experimental results obtained from backward pump fiber amplifier was tested. Deviation of these two results is less than 10%. The results show that how the extraction efficiency of fiber amplifier is impacted by those factors and these influences could be beneficial to the design and application of continuous-wave high power fiber amplifier.