Abstract: Abstract: Distribution network is the network that distributes electric energy in the power grid. The distribution transformer usually refers to a power transformer that operates in the distribution network and has a voltage level of 10-35 kV and a capacity of 6300 kVA and below that supplies power directly to terminal users. Distribution transformers are an important part of distribution networks. The gas-insulated transformer internally replaces the transformer oil with SF6. Due to this limitation, low-voltage, low-capacity gas-insulated transformers have more mature technology level and wider application field because of their low cost and difficulty in production. Due to its good safety, flexible installation methods, and no need for oil pits and clean oil equipment, it has broad application prospects in the field of distribution transformers. Different types of discharge occur in the solid insulation of gas-insulated transformers. In order to judge the type of discharge in the internal insulation of the gas-insulated transformer and the severity of the fault, three kinds of electrode models were used to simulate the corona discharge, internal discharge and external discharge of the insulation in the gas-insulated transformer. The corona discharge model is a needle plate electric with 20mm distance, the needle-plate electrode against the surface of the intact insulation as the external surface discharge model. Artificially add a crack to the insulator and let the needle-plate electrode discharge along the crack. Use this method to simulate the internal surface discharge. The characteristic decomposition products of the specific discharge form were determined by comparing the different kinds of decomposition products and the content of the decomposition products in the SF6 environment. The discharge form of the gas-insulated transformer were determined for the on-line monitoring. At the same time,we can make sure whether a fault occurs or a crack is appeared on the insulation.This article mainly studies the following aspects： 1 Introduction Introduce the characteristics of gas-insulated transformers and the research status and existing problems of GIT discharge decomposition products at home and abroad. 2 Experimental part 2.1 Introduction of Experimental Platform Introduce all the equipment involved in the experiment, the design method and model diagram of the three electrode models, the experimental circuit schematic and the physical map. 2.2 Experimental process Experimental procedure and experimental parameter setting. 3 Experimental results and their processing 3.1 Gas Component Monitoring Results and Data Processing The composition and content of the decomposed gas under various discharge patterns were obtained, and the line graph of the relationship between gas concentration and time was plotted, the growth and saturation trends of each component were analyzed, and the reason was analyzed. 3.2 PD signal monitoring results At the same time, the partial discharge signals under the three discharge patterns are monitored and the respective average discharge capacities are calculated. 4 Experimental results and analysis From the reaction process of the entire experiment, the causes of various gases were analyzed. Errors and factors that cause errors. 5 Conclusion The characteristic gas of the inter-surface discharge and the outer surface discharge with respect to the corona discharge, and the characteristic gas of the inner surface discharge with respect to the outer surface are obtained. This can be used to determine the type of discharge within the GIT. The relationship between the seven kinds of discharge products involved and the increasing trend of each product with the discharge time.