In this paper, based on laser-ultrasonic conversion technology, the internal defect detection method of SF6 gas-insulated enclosed combined electrical appliances (GIS) is studied. Through the analysis of the operation of GIS equipment in Shenyang Electric Power Bureau, the characteristics of partial discharge, the comparison of the causes of partial discharge and the analysis of the characteristics of laser-ultrasonic conversion technology, the sensor layout, detection process, diagnostic basis and characteristics of partial discharge detection of internal defects in GIS equipment are discussed. The principle is analyzed, and the diagnosis process of partial discharge in GIS based on ultrasonic conversion technology is analyzed. The results show that the maximum position of the signal is 1/4 of the height below the upper flange of the circuit breaker. Based on one circle of the circuit breaker tank, the stable partial discharge signal can be diagnosed. The signal has a small amplitude of 5.5 mV. Based on this position, the amplitude attenuation trend of the downward signal and the upward signal is obvious. In one circle of the tank, the stable partial discharge signal can be diagnosed, so it can be inferred that the discharge source is not in the shell but in the central conductor. Finally, some suggestions for partial discharge detection and diagnosis in GIS are put forward to provide reference for equipment maintenance strategy.