针对非弱导条件下单模多模单模（SMS）结构光纤段的理论分析，研究了简单准确的数值模拟方法。利用输入场的泰勒级数展开和多模光纤内电磁场的矢量模式展开，分析了在非弱导条件下SMS结构光纤的多模光纤段中所有模式的传播特性和透射率。基于对模式特征方程的分析，通过合理选择对SMS结构光纤的多模干涉特性有贡献的最少的模式数，计算出每个模式的透射率并与实验结果比较，证明了在单模光纤和多模光纤纤芯精确对准的情况下，利用线偏振径向函数即零阶贝赛尔函数展开多模光纤内的电磁场（标量模型）可以得到很好的数值模拟结果。该结论可有效降低理论研究的复杂性。

Aiming at theoretical analysis of single mode-multimode-single mode(SMS) fiber under unweak-guidance condition, a simple and accuracy simulation method is studied. Propagation characteristics and transmission efficiency of all the modes of mutimode fiber segment in SMS fiber under unweak-guidance condition are analyzed using Taylor′s series expansion of input light field and vector mode expansion of electromagnetic field in multimode fiber. Based on analysis of mode characteristic equations, the smallest mode numbers contributing to multimode interference in the single mode-mutimode-single mode fiber are reasonably selected and transmission efficiency of every mode is calculated and compared with experimental results. It is proved that the light field in the multimode fiber can be expanded only by the linearly polarized radial function, zero-order Bessel function(scalar model) to obtain the good simulation results under condition that the single mode fiber and multimode fiber are well concentric. This work can simplity complexity of theoretical research in this field effectively.