Abstract: The paper analyzed the flowing conditions of gas in coal-based pore and fractured medium,discussed the basic movement equation of gas flow and diffusion in the pore and fractured medium as well as the main influence factors,analysed the coal and gas outburst mechanism and its dynamic conditions.It was recognized that gas flowing is the preliminary condition of coal and gas outburst,the gas pressure gradient is the basic force of coal and gas outburst.Coal and gas outburst was closely related to the gas pressure gradient instead of gas pressure.The coal and gas outburst conditions are mainly determined by gas pressure gradient and coal shear stress.It is easier to form coal and gas outburst in fractured coal due to its low permeability and high gas flowing resistance.