A kind of relative humidity sensor based on March-Zehnder (MZ) interference is proposed in this paper. The sensor is composed by splicing with a waist enlarged single mode fiber (SMF) with a length of 15 mm between two SMFs, whose surface is coated with a thin layer of 5% polyvinyl alcohol (PVA). With the humidity changing, and the effective index of the cladding mode changes. When the relative humidity changs, the central wavelength of the interference peak shifts, so that the relative humidity can be obtained by measuring the sensor′s transmission spectrum. The sensitivity of the proposed humidity sensor is 0.0983 nm/% RH measured by experiment. Experiments also show that the MZ-based humidity sensor has a good stability and a series of other outstanding advantages.