Much electrical equipment works in a closed or semi-closed space for a long time. Partial discharges often occur, and lead to the changes of air composition in the partial discharge area. To study the change law of air components in the closed space when the partial discharges occur, this paper designs three different defect models to simulate common failure types of electrical equipment, and builds a partial discharge experiment platform calibrated based on electrochemical detection technology. Then it analyzes the change law of air composition under continuous discharge conditions, and uses the discharge volume in the partial discharge process to represent the discharge stage. Finally, the relationship between the concentration of air component, voltage, discharge volume and discharge time is established. This provides a theoretical basis for early warning of electrical equipment failure. The experimental results show that the elements of C, N and O participate in the ionization process when the air in the closed space decomposes. The S element does not participate in the reaction. The CO, NO2 and O3 change significantly after a partial discharge. The gas concentration, output rate, and growth trend are related to the voltage and discharge volume when the partial discharge is stabilized in different defect models. This work is supported by the National Natural Science Foundation of China (No. 51707135) and the Science and Technology Support Program of Hubei Province (No. 2015BCE074) and the Science and Technology Project of State Grid Zhejiang Electric Power Company (No. SGZJPX00PGJS).