Based on the theoretical analysis for realizing the Raman gain spectrum flattening technique, the constraints of Raman gain spectrum flatness were improved. Matlab was used to analyze the effects of fiber loss on the flatness of gain spectrum of RFA. The results show that: in the Raman fiber amplifier system, different loss coefficient between each light in C-band is one of the key factors of gain flattening, which means different loss coefficient between the signal lights can cause the deterioration of the gain flatness of Raman fiber amplifier, the loss coefficient difference of each signal light is bigger, and the gain flatness of Raman fiber amplifier is greater.