Ytterbium-doped fiber amplifiers (YDFAs) are widely applied in the fields of industrial processing, radar detection, medical treatment, and communications due to its simple energy level structure and high conversion efficiency. However, the performance of a YDFA is affected by many factors. Particularly under the influence of reabsorption effect, the signal power will increase first and then decrease as the length of the gain fiber increases. In this work, a simulation model of a YDFA is established, and the amplification processes of the signal light, pump light, and amplified spontaneous emission in Yb-doped fiber are simulated and calculated. In addition, the influencing factors affecting the performance of the YDFA are analyzed. The internal relationships between the pump method, pump power, peak wavelength of the signal light, type of gain fiber, and amplification performance are discussed herein. This study can provide theoretical support for the optimal design of YDFAs and further power scaling.