Abstract: Spontaneous emission in EDFAs is amplified with signal, constructing amplified spontaneous emission. In order to analyze the noise characteristics of EDFAs, the output power of ASE must be calculated. In this paper, the relation equations between ASE and signal optical power are derived. When EDFA is used as the power booster, the ASE output power of EDFAs can be calculated simply by this method because the longitudinal distribution of signal optical power in EDFA can be calculated by numerical solution of the optical power rate equations of EDFA.