The increase in moisture concentration within gas-insulated switchgear (GIS) reduces the insulation level of the equipment, necessitating the detection of SF6 moisture concentration. Addressing the challenge of the traditional dew point method being impractical for in situ and long-term online operation, this study proposed a fiber moisture sensor based on tilted fiber Bragg grating (TFBG) coated with multilayer graphene oxide (GO) films. An improved method of heat deposition coating was developed to achieve multilayer deposition of GO on the TFBG surface. By increasing the number of coating layers, the sensitivity of moisture detection was enhanced. An experimental platform for SF6 moisture detection was established to validate the sensitivity, hysteresis, and repeatability of the proposed moisture sensor. The results indicated that the sensor exhibited a response time of 6 min, with a moisture detection range of 13–1081 ppm. Within the linear response range, the sensitivity reached 8.6 pm/100 ppm. Additionally, the sensor demonstrated good hysteresis, repeatability, and reliability. Thus, the proposed fiber moisture sensor in this article enables in situ detection of SF6 gas within GIS.