A fiber sensor which can measure the temperature and refractive index simultaneously is fabricated based on an all-fiber Mach-Zehnder interferometer (MZI). This MZI is formed when a section of a hard plastic cladding fiber (HPCF) is spliced between two sections of single-mode fiber (SMF). Because of mismatching of the core diameters of SMF and HPCF, interference spectrum is generated by the transmission light from the HPCF. As the interference spectrum varies with the change of ambient temperature and refractive index, the simultaneous measurement of temperature and refractive index is realized. Experimental results show that the sensitivity of temperature and refractive index can achieve to 13.3 pm·℃-1 and 52.56 nm·(RIU)-1, respectively. This optical fiber sensor has the characteristics of small volume, simple structure, easy to fabrication and high sensitivity, which is suitable to operate in electrical noisy surroundings.