To discuss the mechanism of gas pressure in coal and gas outburst process，with the constant of moulding pressure，moisture and forced status of the outburst coal sample, five outburst experiments of the different gas pressure are simulated by the independently developed coal and gas outburst simulation test device and the same coal sample. It is found that coal and gas outburst could produce pyriform or elliptic holes with small mouth and big belly. And the volume of hole to the volume of outburst coal is from 1/2 to 2/3. Moreover, there is a threshold value of gas pressure for coal and gas outburst and greater with the increase of outburst intensity. As the force of outburst, the gas pressure has a certain effect of ejection and porphyrization on the outburst coal powders. The coal sample temperature varies continuously with the trend of rising firstly and decreasing subsequently. And the grads of coal temperature can be adopted for predicting the coal and gas outburst.