Abstract: The gas-insulated metal-enclosed switchgear (GIS) is an indispensable electrical device in the electric power system, but there still needs to be more reliable means to monitor the weak defect discharge on the surface of the basin insulator. The optical measurement, as a non-electrical detection method, has a good effect on discharge detection in low-light environments of GIS. In this paper, the LED light source is used to verify the accuracy of the optical path-tracking simulation. Secondly, based on simulation and experiment, the propagation law of the surface defect discharge light signal of insulators in GIS is studied with the change of distance, and the requirements and arrangement method of an optical sensor for detecting the weak discharge in GIS are proposed. Finally, the detection sensitivity of the optical measurement method for weak discharge signals is explored. The results show that the optical path tracking simulation shows the same rule as the LED light source test, and the maximum error is less than 10%. With the increase of the distance of the sensor, the light signal of the front and side defects of the insulator has an obvious attenuation law. In contrast, the light signal of the back defects has no attenuation law. The wavelength requirement of optical sensor detection is 300−550nm, the minimum detection light intensity should be greater than 1.43×10⁻¹¹W, and the response time should be less than the single pulse time of partial discharge. The photomultiplier tube arranged in the middle of the inner wall of GIS has the best detection effect on the defects at different positions on the surface of the basin insulator. For detecting weak discharge, the sensitivity of light measurement is higher than that of the pulse current method. The research results of this paper have reference value for the application of optical sensors in detecting the internal defect discharge of GIS.