The feasibility of applying mode propagation analysis (MPA) method in solving single mode- multimode- single mode (SMS) fiber structure is confirmed by comparing the experimental data with the theoretical results. The method of searching for linear response points on SMS based on MPA was proposed, applied to finding a wide-range high-resolution linear response wavelength theoretically, and confirmed in experiment. By using a 104 μ-in-diameter no-core fiber as short as 4.4 mm, we archived a linear transmission variation of 19.10 dB in the refractive index range of 1.326–1.417, which means a refractive index resolution of 4.76×10-5 according to the resolution of common commercial optical powermeter, and realizes a wide-range high-resolution linear response under the direction of the theory.