Very fast transient overvoltage（VFTO） has much threat on insulation of primary equipment and secondary device in gas insulated substation.Arcing model is one important factor to affect simulated results of VFTOs.In this paper the effect of arcing model on distribution of VFTOs were studied based on 3D transient electromagnetic simulation model.The initial conditions for 3D transient electromagnetic simulation model were discussed based on vector dynamic potential.Comparisons of VFTOs at different positions along GIS duct were taken.From the results simulated we can see that the peak value of VFTOs at one arcing end is affected largely when the radius of arcing channel decrease to 0.03 m.And the time lags of VFTOs at the same point along duct are occurred with decrease of arcing radius.Effect of radius of arcing channel is basically the same for three different points in GIS duct.This study is beneficial to modeling for VFTO analysis in GIS.