In this paper,we present a set of rate equations for analyzing the bidirectional signal propagation in Erbium-Doped Fiber Amplifiers (EDFA), which takes the fiber background loss into account and neglects the effect of Amplified Spontaneous Emission (ASE).The equations are obtained under the condition of low signal gain and suited for unidirectional or bidirectional pumping schemes, respectively. Using these equations, we have analyzed theoretically the bidirectional signal gain properties of a Distributed Erbium-doped fiber amplifier (DEDFA), in the case of bidirectional pumping scheme, we show that it is important for the bidirectional signal propagating model to flatten the gain spectra of distributed erbium-doped fibers (DEDF).