Abstract: The energy change in the course of outburst is divided into three stages, it is pointed out that the greatest effect on environment is throwing power and airflow impact energy after outburst, and these energy are the parts of gas expansion energy which can be estimated by gas expansion energy. Then the existing calculation formulas of gas expansion energy were analyzed, the impact of coal wall gas emission is ignored for the existing calculation formula, and the calculated results include the parts of the dissipation energy. Finally, the method is put forward that the gas expansion energy is estimated by the assuming thermodynamic process and it is analyzed by calculating.