A hybrid-Fabry-Perot (F-P) interferometer based on an in-fiber ellipsoidal cavity is presented, and the refractive index sensing properties are studied. The ellipsoidal air-microcavity is formed by splicing together a single-mode fiber and a photonic crystal fiber with special arc-discharge technique. The cavity loss is analyzed by using a Gaussian beam model and the ABCD law, and the physical model of electromagnetic transmission is established. According to the cavity length ratio, there are two kinds of the influences of environment refractive index on interference fringe: contrast modulation and wavelength modulation. A fiber refractive index sensor with an enclosed air cavity based on wavelength demodulation is proposed in this paper. The result of simulation shows that the sensors has no turning point in a range of 1-1.6. A wavelength interrogation technique is used to demodulate refractive-index with high sensitivity (～ 37.088 nm·RIU-1) and high resolution (～ 2.69× 10-5) and with low temperature crosstalk. Experimental results are in good agreement with the theoretical ones. The F-P fiber sensor also holds advantages such as compactness, low cost, easy fabrication, high contrast, high resolution, no turning point, and low temperature crosstalk.