Abstract: In order to research the interrelation between coal rank gas diffusion initial velocity and coal firmness coefficient, this study adopted the on-site coal sampling coupled with lab testing, and obtained 760 different coal samples about volatile, ash content, gas diffusion initial velocity and firmness coefficient. The results show that the initial velocity of gas diffusion is decreased as the metamorphic grade is decreased; the coal hardiness coefficient is decreased as the metamorphic grade is increased. The risk degree of the coal and gas outburst of different coal rank identified by this two index is anthracite, low volatile coal, middle volatile coal, high volatile coal in order. The existence of ash will reduce the risk of outburst because of the increased firmness coefficient and the decreased initial gas diffusion velocity △p.