In order to improve the gain flatness and transmission bandwidth of the fiber amplifier in the dense wavelength division multiplexing system, a theoretical model combining multi-wavelength pumping and cascade technology is established based on two non-same fiber types, when different wavelength combination modes are injected at different positions. Simultaneous pumping with multiple pumps can broaden the bandwidth, and flat gain fluctuations in fiber cascades. We use the fourth-order Runge-Kutta method to find the numerical solution of the N-channel Raman coupled wave equation. The Fourier curve with high fitting degree is used to fit the simulated spectral lines. Therefore, the Fiber Raman Amplifier (FRA) can be amplified first and then compensated. The transmission bandwidth and average gain value of the signal light are improved, and ideal gain flatness can be maintained without using a gain equalizer. The simulation results show that the final amplifier has an average gain of 36.75 dB, a transmission bandwidth of 57 nm, and a gain flatness of only 0.03 dB, which can provide a theoretical reference for the actual FRA design.