The　extinction　coil　of　automatic　tuning　is　important　in　reducing　grounding　current,　depressing　flash　and　grounding　surge.　Before　bringing　into　the　electric　grid,　the　equipment　must　be　tested　for　the　safe　and　equilibrium　operating.　In　the　article,　through　the　simulation　experiment　on　the　magnetic　biasing　extinction　coil,　verified　the　validity　of　high/low　voltage　regulating　characteristic　for　the　all-digital　instrument,　and　analyzed　the　characteristics　to　judge　this　equipment　whether　can　meet　the　requirements　of　grid　system　operation.　The　results　showed　that　the　extinction　coil　of　automatic　tuning　can　play　a　role　when　earth　fault　occurs　and　the　actual　regulating　characteristic　can　meet　the　practical　requirements.