Abstract: According to the principle of evanescent wave, a humidity sensor based on optics in-fiber Mach-Zehnder interferometer was proposed by using the standard single mode fiber with two waist-enlarged bitapers, and the center of sensing arm was tapered by adiabatic flam-heated fiber tapering machine. The light propagates from the input end of the sensor, the first bitaper excites a number of higher-order modes, each mode transmits through tapered fiber and into the second bitaper, which can be coupled into the output fiber of the sensor. The change of the humidity can lead to the evanescent field of the tapered fiber changing, eventually causes the energy of the transmission spectrum energy to change. The environment humidity sensing measurement can be realized through measuring the variation of transmission spectrum energy. The experimental results show that within the humidity changing scope of 35%~85%, the energies of the transmission spectra have the same variation trend, a valley of interference spectrum near the water vapor absorption peak has the highest sensitivity of 0.157 dBm/%RH, the temperature cross-sensitivity is as low as 0.014 %RH/ ℃. This humidity sensor is advantaged by easy manufacturing, high sensitivity and immunity of temperature cross-sensitivity, it have a great application prospect.