Abstract: In order to realize the flat inline amplification of all dense wavelength division multiplexing(DWDM) transmission channels in optical fiber communication systems, it was proposed to get gain-flattened Raman fiber amplifier based on cascading highly nonlinear fiber. An approximate value of output signal power at the output port was finally obtained considering different signal light losses, when the linear method was used for obtaining the Raman gain spectrum. Moreover, the average gain and gain flatness were 20.45 dB and 0.15 dB, respectively. Then, the factors for the output characteristics of Raman fiber amplifier were analyzed. Simulation results demonstrate that the designed Raman fiber amplifier has advantages of high gain, smaller gain flatness as well as simple configuration.