Abstract: It is important to study initial discharge for researching operating process in gas spark gap. The whole initial discharge process in gas spark gap was simulated in this paper by PIC-DSMC code. The evolutions of electron density distribution and ion density distribution were obtained, the electric field distribution in the gap was analyzed, and the process from electron emission to plasma channel was presented. Finally, the gas spark gap discharge experiment was performed with high speed camera. This is basis for further researching on physics mechanism of the gas spark gap.