放大器的增益谱平坦性是波分复用系统设计中一项十分重要的参量.本文推导了N段光纤级联实现增益谱平坦的分析理论，得到每段光纤的有效作用距离和喇曼功率增益系数的约束条件.当喇曼增益谱拟合为线性时，简化了其实现增益谱平坦的条件，并给出了两段光纤级联信号光获得增益的最优值.以纯石英光纤和掺磷石英光纤级联的模型为例，利用Matlab进行了数值分析验证，结果表明：随着第一段光纤长度的增加，信号光获得的增益先增加再减小；当第一段光纤取7.15 km时，信号光获得了5.78 dB的平均最大增益和0.45 dB的增益平坦度，并得到泵浦光功率与信号光增益呈正相关性.最后，以三段不同材料光纤级联为例，利用Matlab进行了分析验证，信号光经过放大后获得了7.74 dB的平均增益和0.57 dB的增益平坦度，证明了理论分析的可行性.该方案为增益平坦化的喇曼光纤放大器设计提供了一种新的思路.

Gainflatness is a crucial parameter in wavelength division multiplexing system design. In this paper, the gain flattened theory by using multistage cascaded fiber was presented. The effective distance and Raman gain coefficient constraints were calculated. When spectrum was fitted by linear lines, the gainflattened condition was simplified. The optimal gain value was obtained by cascading two stage fibers. The pure silica fiber and phosphorusdoped fiber were cascaded as the typical example. The results show that with the increasing length of the first stage fiber, the signal gain was firstly increased and then reduced; when the first fiber length was 7.15 km, 5.78 dB average maximum gain and 0.45 dB gain flatness were provided by Matlab. Pump power and signal gain was positively correlated. Three stage fibers were cascaded with different materials as the last example. 7.74 dB average gain and 0.57 dB flatness were verified by Matlab which proves the accuracy of the theory. This scheme provides a novel way to design gain flattened Raman fiber amplifier.