An optofluidic refractive index sensor based on large air-hole polarization-maintaining microstructured optical fiber polarization rocking filter (PM-MOF-PRF) is proposed. Resonant coupling between orthogonal polarization modes occurs when a periodic back-and-forth twist is induced by PM-MOF along the axial. Via polarized detection, a transmission spectrum similar to a long-period fiber grating can be observed, and thus a polarization rocking filter (PRF) is obtained. The transmission spectrum of this device is simulated based on mode-coupling theory. When splicing a short piece of C-shaped fiber between the PM-MOF and single-mode fibers (SMF) at both end of the PRF, it allows fluids running into and out of the air hole channels without affecting light coupling between the MOF and SMF. Therefore, an all-fiber optofluidic refractive index sensor can be constructed. Then the phase modal birefringence dispersion curves of the PM-MOF for microfluidics with refractive index around 1.333 is simulated by finite element method, and the transmission spectra of the PRF for different microfluidic refractive index values are obtained. By tracking the wavelength shift of the spectra, a refractive index sensitivity of 7196.4 nm/RIU is achieved, and the value can be improved to 16754.0 nm/RIU when the MOF is tapered to half of the initial diameter.