An apporoximative theoretical of an erbium-doped fiber amplifier (EDFA) has been proposed in this paper by the rate equation of three-level system. The optimum fiber length is obtained with different pump wavelength by considering the excited-stated-absorption(ESA) in EDFA. The relations between the gain and signal light intensity and pump light intensity are analyzed. It is found that the gain spectrum shape can be changed with the fiber length, which provides the evidence that EDFA is of great potential use in wavelength division multiplexed telecommunication systems.