Abstract: Referring to roadway butterfly shape rock burst mechanism and nonlinear dynamic system theory,a conjecture about butterfly-shape coal and gas outburst mechanism in excavation roadway has been made,In this conjecture,the mechanical essence of coal and gas outburst in excavation roadway is that a certain degree of butterfly shape plastic zone increment appears in front of excavation face in an extremely short period of time,triggering in the quick release of surrounding rock elastic energy and gas energy,thus resulting in coal and gas outburst.From the aspects of consistency of plastic zone changing laws between butterfly rock burst and coal and gas outburst,the outburst energy source and the rationality of conjecture was verified,and the risk evaluation system of coal and gas outburst was established as well.Four necessary conditions of strength,stress,angle and triggering event for coal and gas outburst were put forward,besides,the sufficient conditions of coal and gas outburst were determined; Quantitative analysis and explanation(not qualitative)of the phenomenon,occurring condition and process of coal and gas outburst were studied for the first time.The research result has provided a new idea for the research work in the field of coal and gas outburst forecast,prediction and prevention in coal seam roadway.