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.