Based on the theories of arc and gas discharge, a novel hybrid arc model in vacuum circuit breaker is proposed, considering the factors influencing on the arc formation from both microcosmic and macroscopic views. The evolution from initial discharge channel forming with breakdown to steady arc state is simulated. The results show that the radiuses of discharge channels are enlarging and the shapes of arc are changing from diffuse mode to contractive mode with the increasing of the current applied, but the shapes remain the diffuse mode as a whole. Along the axial direction from the cathode to anode, the electron density is increasing, and the average velocity of electron is increasing at first but decreasing later. Further more, the thermal inertia of arc is verified.
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