The coal and gas outburst accident on May 20, 2007 at Sihe Coal Mine is unique in that there is no or very little soft coal in the coal seam and that the coefficient of coal strength is very high. It was also indicated in the outburst that there is a very high gas content of 20-30 m3/t in the coal seam nearby the outburst site. Based on field test and observations, together with laboratory experiments, detailed analysis was conducted on controlling factors of coal and gas outburst potential at Sihe Coal Mine. Stress data available from Sihe Coal Mine, northern China and other parts of world, were also taken into account to facilitate the analysis. The results show that the stress level is not abnormal compared to other coal fields in China prone to outbursts, and that the gas contained in the coal seam is the main factor that triggered the outburst. The results show that outbursts are limited to only a small part of the whole coal seam and that the East part of coal field is not prone to outbursts.