The invention belongs to the technical field of sensing, and provides an on-line dualbeam interference-type fiber refractive index sensor. The dualbeam interference-type fiber refractive index sensor comprises first single-mode fiber, light splitting fiber, offset-welded sensing single-mode fiber and reference single-mode fiber, light composition fiber and second single-mode fiber; while measuring, the sensing single-mode fiber is positioned in a to-be-checked substance; the wide spectrum light is input from the first single-mode fiber, then is coupled to the sensing single-mode fiber, and is subjected to propagation by a fiber core mode and a cladding mode and then mode transition, the fiber core mode is coupled to a cladding layer for continuous propagation, the partial energy of the cladding mode is coupled in the fiber core for continuous propagation, the light output by the reference single-mode fiber is subjected to light composition interference in the light composition fiber and is output through the second single-mode fiber, wavelength demodulation is carried out, and the refractive index change is obtained. Due to the light path compensation of the reference single-mode fiber, interference fringe drift distance is large, sensitivity is higher; the cooperation of the sensing single-mode fiber and the reference single-mode fiber have temperature compensating effect, so that the interference immunity and detection precision of the sensor are increased.