We propose a simple and low-cost Fabry-Perot (F-P) fiber sensor based on an inner air cavity. The air cavity is fabricated at the fiber-tip by splicing a single-mode fiber and a hollow-core photonic crystal fiber (HC-PCF), with un-collapsed section of the HC-PCF being cleaved. Application of F-P fiber-tip sensor in the external refractive index (RI) measurement is experimentally demonstrated. The sensor exhibits a good linear response and high sensitivity of -54.409 dB/RIU in the RI range of 1.333–1.413. Therefore, it is applied to fiber-optic biological and chemical sensing.