This study presents a microfiber-based Mach-Zehnder interferometric optical structure. The fiber-optic humidity sensing, which employs the wavelength corresponding to the interference peak as the sensing parameter, is realized using a thermal reduction method. The thermal reduction method changes graphene oxide (GO) film, which precovers the fiber, into a reduced graphene oxide (RGO) film. The sensor can achieve a maximum average sensitivity of 0.2768 nm/%RH in relative humidity range of 45%-95%. The response and recovery time of the sensor in humidity sensing are 6 s and 30 s, respectively. The humidity response and temperature response of the sensor exhibit different characteristics, and the sensor has good time stability and recoverability in humidity sensing. This study provides a novel interference-type graphene-based fiber-optic sensor fabrication method using the microfiber-based interference structure and process engineering that changes GO into RGO.