The gas-insulated switchgear (GIS) is increasingly used in power systems because of its small size and high reliability. The most common problem in using GIS is partial discharge. To prevent it, insulation is required along with early detection in GIS. The defects inside the GIS need to be found in time to prevent major accidents due to partial discharge. Therefore, we studied the partial discharge detection of GIS to ensure the safety and reliability of GIS operation. Detecting partial discharge also helps to inspect and repair GIS appropriately. In this study, the excitation, transmission, and leakage of electromagnetic waves excited by GIS partial discharge were analyzed as the partial discharge signal of ultrahigh frequency is propagated, reflected, and refracted in GIS by earthing through the outer surface of the metal casing and generating a transient earth voltage. We designed a transient earth voltage sensor on the basis of the characteristics and requirements of GIS. The sensor is used to find defective GIS in operation. The results of the detection of the partial discharge and the signal measurement of defective GIS are important references for online inspection and monitoring.