Optical fiber has remarkable advantages such as excellent anti electromagnetic interference in monitoring gas insulated switchgear (GIS), showing unique stability and reliability in high voltage environment. In addition, the weak grating array technology based on fiber enables the large-scale deployment of fiber sensors, which has high flexibility and ductility. Therefore, this paper combines weak fiber Bragg grating (WFBG) static leveling instruments and strain sensing optical fibers, to achieve the safety monitoring of GIS. The former realizes settlement monitoring in the vertical direction, whereas the latter achieves displacement monitoring in the horizontal direction. Both the static leveling instrument and strain sensing optical fiber employ weak fiber Bragg gratings as sensitive components and are interconnected using time-division multiplexing, to achieve long-term real-time settlement monitoring of ultra-long GIS equipment foundations. This system is applied to the monitoring of GIS foundation settlement in a ±800 kV ultra-high voltage converter station, comprising 33 WFBG static levels and 325 WFBG strain sensors. The monitoring data obtained over a two-month period align with manual observation data, indicating its excellent practical value in relevant structural monitoring.