This paper reviews a specially designed few-mode fiber and a single mode fiber-few mode fiber-single mode fiber (SFS) sensing structure. Only the fundamental core mode LP01 and the first symmetric high order core mode LP02 are excited in the few-mode fiber under the axial to axial splicing condition, and there is a critical wavelength existing in the transmission spectrum of the SFS structure at the operational wavelength. Near the critical wavelength, the wavelength spacing between adjacent interference peaks reaches maximum, and the output intensity of the interferometer is fixed while the wavelength changes. Therefore, the critical wavelength is exclusive and easy to identify from the transmission spectrum. The sensing characteristics of the critical wavelength and the interference fringes in the transmission spectrum under strain, temperature, bending, and liquid refractive index are studied theoretically and experimentally, and the SFS sensor structure is applied to measure a wide range of sensing parameters including strain, temperature, curvature, displacement, surrounding refractive index, and relative humidity, with the advantages of large measurement ranges, high sensitivity, and multi-parameter measurement. Thus, the current problems of limited measurement range and multi-value outputs existing in the traditional interferometers can be solved.