The outburst coal mass is a geologic body which had objectively existed before outbursts occurred.The authors think the occurrence and development of coal and gas outbursts are controlled by comprehensive factors according to a host of facts of outbursts in their researches,and all the factors belong to the system of "rock coal gas rock".Therefore the authors put forward a completely hypothesis,i.e.key layer and stress dike mechanism of coal and gas outbursts.This hypothesis emphasizes that the outbursts are the result of comprehensive action by coalbed gas,abutment stress and physical and mechanical properties of coal.Of special interest is emphasizing the geologic backgroud and conditions of outbursts to a great extent.Any phenomenon of outbursts which a person could observe could be qualitatively explained with the hypothesis so that the hypothesis can serve the study of geophysical field of outbursts perfectly.