Abstract: An actual SF₆ circuit breaker chamber is adopted as the model in this paper. By gas dynamic equations and collisions between charged particles, the generation, development and variation processes of arc plasma at the break of 3mm are studied from a microscopic view before the breaking of liquid metal bridge and stable arc formation. Effects of the content of metal vapor produced by high breaking current and secondary electron emission coefficient produced by different electrode materials on the microscopic parameters of arc plasma in arc before process are studied here. This simulation can provide the basic theory data for the steady-state arc research, and has certain significance for the development of high voltage switchgear.