A novel all-fiber refractometer sensor is proposed, which is based on multimode interference in the multimode fiber core section sandwiched between two single-mode fibers. A wide-angle beam propagation method in the cylindrical coordinate is employed as the modeling tool for simulation and design of the proposed refractometer sensor. The design for a refractometer is presented that shows that the refractometer would have an estimated resolution of $5.4\times {10}^{-5}$ for refractive indices from 1.33 to 1.45 and of $3.3\times {10}^{-5}$ for refractive indices from 1.38 to 1.45 through the choice of an appropriate length of the multimode fiber core section.