A sensing method of refractive index based on single-mode-multimode-single-mode fiber structure is proposed. The sensor sensitivity is increased through valley detection method. It shows that number of pattern in the multimode fiber has vital influences on the transmission result. When the structure parameters of single-mode fiber and multimode fiber change and normalized frequency remains, the number of pattern of multimode fiber remains, and the regular transmission curve can be acquired. The sensitivity can be improved effectively with the increase of diameter of multimode fiber. The maximum sensitivity is 3.725×105 nm/RIU and the detection limit is 7×10-7 RIU, when the diameter of multimode fiber is 70 μm. When the actual length of multimode fiber is deviating from the design, the length of the deviation has little effect on the sensor sensitivity and detection limit, the output spectrum shift is -0.07 nm/μm, which means it can be practiced in high sensitive stress sensing.