Based on the needle-plate model, the decomposition products and production mechanism under different humidity and partial discharge voltage are analyzed by experiment and theoretical calculation. The Materials Studio is used to study the possible components, and the components production pathway is studied. Through analyzing the reaction heat and the reaction barrier the components involved in, the reason why the CO and NO₂ components is chosen as the characteristic products to characterize the insulation status of switchgear is fundamentally explained. The theory foundation is established for monitoring the insulation status of the switchgear by detecting the air decomposition components. The air decomposition components: detected in the experiment process are CO and NO₂. When the humility keeps unchanged and partial voltage increases from 6 kV to 7.5 kV with 0.5 kV increment, the concentration of CO and NO₂ increased with the increase of discharge voltage. On the other hand, keeping the discharge voltage still and adjusting the relative humidity to 50%, 70% and 90% respectively, the concentration of CO increased with the increase of humility, and the concentration of NO₂ obviously decreased with the increase of humidity. Under a certain voltage and humidity, the concentration of CO shows a linear increasing trend over time, while the increasing rate of NO₂ gradually reduces over time and tends to saturate ultimately.