In the research process of very fast transient overvoltage（VFTO）

the establishment of an equivalent model which simulates the breakdown and extinction of arc caused by disconnector switching

has great effect on the result of VFTO calculation. A dynamic arcing model is established using the MODELS of ATP-EMTP. The model is based on the arcing analysis theory of Mayr

and takes the influence of arc extinguishing in the point when current reaches zero into consideration. Moreover

taking a500 kV gas insulated station（GIS） as a calculation example

this paper comparesthe dynamic model with other two models

and analyzes their properties. Since the dynamic model considers the variation of arc resistance

the result shows that VFTO amplitude decreased and frequency in the extinction phase increased

when compared to the other two conditions. By the theoretical analysis

the dynamic variation of arc resistance is taken into account or not

has important significance for the VFTO simulation result.