The operation of an intermode fiber-optic interferometer with “Singlemode-Multimode-Singlemode-Multimode-Singlemode" (SMSMS) structure during its spectral interrogation in the C-wavelength range has been studied. The refractive index n of the external environment can be measured using this device by providing the interference between fundamental and cladding modes in this structure. The theoretical expressions for calculating the spectral characteristic (SC) of the interferometer were presented, the structures’ modes were obtained, the behavior of the SC interference components related to the pairwise interference between the fundamental and cladding modes of a single-mode fiber when n changing was analyzed. The methodological errors of n measuring the external environment were estimated. The minimum error in n determining was 1.7∙10–4 in the n range from 1.32 to 1.36, which corresponds to a 0.13% change in the range of glycerol concentration from 0 to 25% in the water solution.