Given the large structure size and low processing reproducibility of the interferometric tapered micro/nano fiber sensor, this paper proposed a non-adiabatic tapered micro/nano fiber sensing structure. First, the dependence of the sensing sensitivity of the interferometric tapered micro/nano fiber sensor on the waist diameter and the sensing characteristics of different parts was investigated according to the fiber mode theory. Then, mode control in the tapering process of the fiber under the non-adiabatic condition was studied. On this basis, a tapered micro/nano fiber sensor structure was designed and fabricated with a waist diameter of 3.1 μm and a taper waist length of 1.1 cm. Its detection sensitivity of low refractive indexes reached 90250 nm/RIU. This structure, with small size and high processing reproducibility, lays a foundation for miniaturization and integration of sensors and is expected to be used in fields such as biomedicine and environmental monitoring.