In this paper

we cascade two symmetrical slow tapers in a single-mode fiber and remove the coating between two tapers

which achieve a Mach-Zehnder（M-Z）interferometer.The cladding is act as a sensing arm

which is sensitive to the refractive index in the outside world.Through the analysis of the optical field evolution of taper region

mode field radius will reach maximum value of 14.8!m when fiber taper has a radius of 25!m.When taper waist radius is 12.5!m

transition region is 5 mm

and taper region interval is 131 mm

the effect of coupling and waving is best.The sensing arm can detect alcohol concentration in the range of 0

1

00%

which has a good linear relationship between refractive index and wavelength offset with detection sensitivity of-120nm/RIU.The sensor has advantages of low insertion loss

stable performance and strong anti-interference capacity

which can be applied to areas such as distillery and water quality monitoring.