A sandwich-structured all-fiber Mach-Zehnder refractive index sensor based on a spindle-type air cavity was designed. The spindle-type air cavity was manufactured by splicing and taper acommon single-mode optical fiber and a photonic crystal fiber. The spindle-type air cavity and cladding in the taper area serve as the referring and sensing arms respectively, then the Mach-Zehnder interference is established. The interference fringes of the sensor and the electric field distribution in the taper area were simulated respectively based on FDTD Solutions and COMSOL. The relationship between the trough wavelength of the index sensor and the environmental refractive index, the effective refractive index and the environment refractive index are obtained by simulations. The sensitivity is 1377.6 and 1436nm/RIU when the refractive index is 1.36~1.37 and 1.37~1.38 respectively.The extremely short interference arm is used to reduce the loss and the sensor has higher refractive index sensitivity.