Dry Wire Electrical Discharge Machining (WEDM) is a newly developed technology that uses gases as dielectric and is in favor of environmental protection. Dry finishing of WEDM offers advantages such as better straightness, lower surface roughness and shorter gap length. Experimental studies show that the machining behaviors in dry conditions are quite different from those in conventional liquid dielectric. In order to explore the processing mechanism of dry WEDM, this paper studies the single pulse discharge mechanism in gas with finite element method. In order to obtain the crater morphology with edge raised, the fluid field was simulated by ANAYS on the basis of thermal analysis of a single pulse generated by the instantaneous discharge in gas.