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A refractive index sensor based on dual-port input of melting tapered fiber coupler is proposed in this paper. By combining theory and experiment, the influence of the coupling degree of the tapered region on the output mode field and output power of the fiber coupler is studied by finite element method. Two single-mode fibers are connected to custom-made couplers with different spectral ratios, and then the fibers are wounded and melted so that the fiber core is refined, closed and tapered. The mode fieldsof the two fibers are coupled and affect each other to realize the fiber transmission of different powers. By using the taper region structure for refractive index detection, the refractive index range of 1.335to 1.399, the sensor sensitivity of 131.29pm / RIU, and the linear fitting degree of 95.98% were obtained. The structure of the sensor is simple, and the input power can be controlled according to the demand. The production cost is low and has a good application prospect and development value.