In-fiber filter has been focused on for many years and has broad applications in fiber optics area. The theoretical analysis of a single-multi-single mode (SMS) fiber based filter is presented. A multimode-measurement is realized based on an interference method in order to prove the excited multimode in the multimode fiber of SMS structure and to provide more accurate parameters used in the theoretical analysis. Designed filters are given to show the validity of the theoretical analysis. Other factors influencing the performance of the SMS based filter are discussed at last.