Gas Insulated Switchgear (GIS) is an inbuilt electrical equipment in the power system, although there are currently insufficient monitoring techniques for the week discharge defects on the surface of the basin insulator. As a non-electrical detection method, photometry has a good detection effect on discharge in the weak light environment inside GIS. This paper uses optical simulation and experiments to study the propagation law of the optical signal of the defective discharge on the linear GIS as it changes with distance and proposes the requirements and arrangement method of the optical sensor for detecting weak discharge inside the GIS. Then, the effectiveness of the sensor layout strategy was verified by an actual 220 kV GIS partial discharge optical detection experiment. The research results show that the optical signal of the partial discharge of the top insulator in the linear GIS is severely attenuated during the propagation process, so a single photoelectric sensor should be arranged in the middle of the straight cavity. In the L-type GIS, the light is concentrated at the corner, so a single photoelectric sensor is arranged at the corner. The lowest detection light intensity of the photomultiplier tube should be greater than 1.43 × 10–11 W, the photomultiplier tube arranged in the middle of the inner wall of the GIS has the best detection effect on defects at different positions on the surface of the basin insulator. For the purpose of detecting internal defect discharges in GIS, the research conclusions of this paper are useful for the application of optical sensors.