In order to reduce the thickness of fiber cladding and improve the refractive index sensitivity of the interferometer, a simple photonic crystal fiber (PCF) Mach-Zehnder interferometer which is easy to fabricate is developed. First, a PCF is fused between two single-mode fibers; then, in a self-made etching tank, hydrofluoric acid is used to reduce the thickness of the cladding. The effects of the length, corrosion time, and ambient temperature on the sensitivity of PCFs are studied by controlling the above variable factors. The results show that the sensitivity increases with the increasing length of the optical fiber, and increases about three times after 40 min corrosion in hydrofluoric acid solution with mass fraction of 40% for 3 cm PCF. The ambient temperature has little influence on the sensitivity of the PCF Mach-Zehnder interferometer.