Based on the CO_2laser fusion technology and mode interference princip le,a collapsed photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated.By changing the structure of PCF′s cladding air holes,the Mach-Ze hnder interferometer is realized by CO_2laser fusion method,and the interference happens betwe en cladd ing mode and core mode.Using the microscope to measure the PCF sensor,it can be obtained that th e length of collapse area is 419.01μm,and the smallest diameter of this area is 58.69μm.The inter fe rence fringe contrast of the transmission spectra can be up to 10.6dB and the temperature sensor and refract ive index sensing characteristics are studied and analyzed.The fabricated Mach-Zehnder interferom eter based on PCF is placed in the temperature control cabinet and glycerin aqueous solution with different refractive indices respe ctively,with both ends of it connected with an amplified spontaneous emission (A SE) source and a spectrometer,respectively.The output power of light source is 16dBm.Therefore,a transmission spectrum can be obtained.Moreover, the temperature and refractive index sensing characteristics are studie d.The experimental results show that the temperature sensitivity is only 0.0051nm/℃ in the range of 30-70℃,and the refractive index sensitivity is 33.3884nm/RIU in the range of 1.3525-1.4134.