In allusion to the “5·30”especially serious coal and gas outburst accident of Tonghua coal mine of Songzao Coal Electricity Group CO., LTD in 2009, a detailed simulation of the mutation process of mine ventilation system during gas outburst disaster period is done by using theory of ventilation network with sources and numerical simulation technology (we develop a computer simulation system). Three changing processes, namely outburst overflow, natural flow and ventilation downstream occurs in the ventilation system after outburst. Due to the expanded power of outburst source, gas enters main ventilation roadways at first, and flows outward, and flows counter-current along main intake air roadway, and then enters low air quantity roadways with air doors or underground chamber, which leads to gas concentration overrun and threat to the whole mine. After outburst, gas power disappears. Due to the action of gas natural ventilation pressure, mine ventilation may begin to flow naturally. After the ventilation systems are restored, gas flows with the wind as a result of the power of the main fan. Due to the interference of gas natural ventilation pressure, the operating points of main fans may drift (relative to roadway structure), but the operating points recover with the discharge of gas. It is worth noting that the detention time of gas in low air quantity roadways and underground chambers is relatively long, and the gas becomes continuous gas supply sources, so it is dangerous. In addition, the delayed pour between main roadways and by-pass low air quantity roadways and the gas natural ventilation pressure caused by gas detention may lead to airflow disorder of reversion in some local circuits, and may enlarge the affected range of gas, or lead to the delay and detention of gas discharge.