A macrobending-based all-fiber refractometer sensor with a simple optical configuration is proposed and investigated both theoretically and experimentally. The proposed fiber refractometer sensor consists of a single-loop structure of bare macrobending standard single-mode fiber (SMF28) with a selected bending radius and reduced cladding diameter. The well-known scalar approximation theory is employed to theoretically predict the characteristics of the proposed fiber refractometer sensor. An approach to improve the resolution of the refractometer is presented, which shows that the refractometer with a reduced cladding diameter of $81\mathrm{\mu m}$ has an experimentally verified resolution of $5.75\times {10}^{-5}$ for a refractive index range from 1.4586 to 1.5396 at the wavelength of $1550\mathrm{nm}$ .