Abstract: Sulfur hexafluoride (SF₆) is a gas used in electrical substations at electric power plants as an insulator in circuit breakers and switchgear. SF₆ gas can generate a variety of decomposed products due to discharge or overheat inside the high-voltage devices. The discharging fault inside of the electric device insulated by SF₆ is closely related to the composition and content of SF₆ gas discharge products. Some of the byproducts are active and unstable，which create difficulties on testing them and have to be analyzed through a variety of testing methods. Gas chromatography，gas chromatography-mass spectrometry and infrared spectrometry have been developed to rapidly analyze the discharge products, but these methods are time-consuming and very hard to realize the in situ analysis and on-line diagnostics. An online monitoring system for discharging products of SF₆ with photoelectron ionization (PEI) time-of-flight mass spectrometry was developed in this paper. It can be used for diagnosis of discharging fault inside of the electric device by real-time monitoring for discharging products of SF₆. The mass spectrum of SF₆ gas discharge products and the kinetics were investigated using a gas discharging study with home-made low direct current voltage system. The main byproducts of SO₂F₂, SOF₂, SO₂ and SF₄ can be clearly detected. This method is expected to provide the power production field with online mass spectrometry monitoring and track the fault inside of the SF₆ electrical equipment on its running condition.