In this paper,an all-fiber Mach-Zehnder interferometer with "single mode fiber-multi mode fiber-multi more fiber-multi mode fiber-single mode fiber" is designed.In this structure,th e multi-mode fiber acts as a coupler.When different modes of light are transmitted in the multi-core fiber, the optical path difference will be produced and the Mach-Zehnder interference will be formed.When the amb ient temperature and refractive index change,the refractive index and temperature can be measured by analyzing the drift of different resonance peaks in the transmission spectrum of the interferometer.Th e experimental results show that the highest temperature sensitivity of the sensor is 46.0pm/℃ at low temperature,109.0pm/℃ at high temperature,and the highest sensitivity for refractive index of the sen sor is 54.3nm/RIU (RIU is a unit of refractive index).In addition,by simultaneously monitoring the shift o f the two resonant peak wavelengths of the transmission spectrum with ambient temperature and refractive index,the simultaneous measurement of ambient temperature and refractive index can be achieved without cross-sensitivity.The sensor has the advantages of simple structure,easy fabrication,good repeatabil ity,stable response and multiplex function.It has a wide application prospect in the field of sensing.