In order to meet the requirements of ultra-high speed and large-capacity optical fiber transmission systems,in the paper the Raman gain spectrum of a photonic crystal fiber based on the structure of two fiber cascades is analyzed,a Gaussian curve is used to fit Raman gain spectrum and a Raman fiber amplifiers with high gain and wide bandwidth is designed.The Gaussian curve fitting preserves the gain spectrum information of the photonic crystal fiber completely and accurately,and increases the signal transmission bandwidth as possible.Using the fourth-order Runge-Kutta method to numerically solve the classical differential equations of the coupled Raman wave,not only reduces the gain flatness of the amplifier,but also achieves high gain amplification.Compared with the method of fitting the fiber Raman gain spectrum linearly,the system transmission capacity is improved.Through simulation analysis,the amplification bandwidth of the amplifier is 61 nm,its gain is up to 22.8 dB,and the gain flatness is only 0.42 dB.