Sulfur hexafluoride（SF

6

）is one of the most commonly used insulating media in high-voltage electrical equipment because of its excellent insulation. Among electrical discharge faults

spark discharge is one of the most common types

often induced by poor contact of fixed metal shield or switch contacts. It is known that SF

6

can decompose to lower fluorides of sulfur in electrical discharge

and these radicals will oxidize in the presence of water vapor or oxygen

resulting in the formation of

decomposition by-products with strong toxicity and corrosivity.This paper proposes a method based on oxygen isotope tracer to observe the variations and to analyze the formations of the oxyfluorides

such as SO

2

F

2

SOF

2

and SO

2

. By injecting different concentrations of18O

2

a series of spark discharge experiments is carried out. Then

the concentration of the oxy-fluorides under different ratios of18O

2

to16O

2

（[18O

2

]

/[16O

2

]

）is obtained. Finally

the formation mechanisms of the oxyfluorides are discussed based on the experiments.