在考虑光纤损耗和级联放大器的情况下，推导了调制不稳定性的产生条件和增益的普适解析表达式，分析了调制不稳定性对信噪比的影响，给出了一个新的计算信噪比的表达式。并利用分立步傅里叶数值方法对非线性传输进行了仿真，证明了本文的解析结论的正确性。 Considering the fiber loss and the cascaded amplifier, we derive the general analytical expression of the modulation instability condition and gain, and analyze the influence of modulation instability on signal-to-noise ratio. A new calculation is given Signal to noise ratio expression. The discrete-time Fourier method is used to simulate the nonlinear transmission, which proves the correctness of the analytical conclusion in this paper.