The fracture propagation in coal-rock under gas pressure is a necessary process for coal and gas outburst.The mechanism of fracture propagation in coal-rock is studied by using fracture mechanics.The expressions of the normal stress and shear stress on the surface of fracture under gas pressure are given first.Then the relationship among the direction of fracture propagation,critical gas pressure and side pressure coefficient is analyzed according to the two situations of tension-shear failure and compression-shear failure.The studies show that for tension-shear fracture,the fracture will be easy to propagate while the fracture has the same direction as maximum principal stress,and for compression-shear fracture,the propagation direction depends on the difference of compression-shear parameter and friction coefficients.