Since the hypothesis of coal and methane outburst mechanisms based on elastic mechanics gives no consideration to time, it cannot give satisfactory explanation of the outburst occurrence and its development. A creep mathematical model of coal containing gas has been set up through creep tests and analysis under a condition of triaxial compression. Such a model can explain the phenomenon of coal and methane outbursts successfully. The tests proved that there is no essential difference between the rheological properties of prone outburst coal and non-prone outburst coal. Outburst can take place in any kind of coal if the rheological conditions of coal containing gas are met. The creep mathematical model provides a basis for establishing a comprehensive index of coal and methane outburst potential.