An in-fiber Mach-Zehnder interferometer (MZI) based on waist-enlarged fibe r taper and graded-index multimode fiber is proposed and demonstrated in this paper.The sensor head is f abricated by fusion splicing a 30mm graded-index multimode fiber with a fiber taper,and the distance between the m is 25mm,thus forming SMF-taper-SMF-GI MMF-SMF fiber structure.The optical fiber ta per can increase cladding mode energy,gradient multimode optical fiber serves as sensing arm,and the changes of the ambient temperature,refractive index and the axial stress will make the sensor core mode and cladding mode of o ptical path difference varied, causing interference sensor spectral change.Monitoring the spectra vari ation can determine the ambient physical parameters.The experimental results show that the temperature sensitiv ity of the sensor is 78.6pm/℃ with a linearity of 0.997in the water temperature range of 30-89℃；the refr active index sensitivity is 81.48dB/RIU with a linearity of 0.989in the refractive index range of 1.333-1.394R I U；the strain sensitivity is 0.33× 10－2 dB/με with a linearity of 0.998in the strain range of 0－933.3μ ε．The sensor has advantages of simple fabrication,low cost,simultaneous measurement of multiple physical parameters.