Abstract: The law of acoustic wave velocity change in coal during loading is studied by the use of self-made acoustic parameter test system. The research results show that during loading with the increase of axial compression or confining pressure, the nonlinear increasing of the longitudinal and transverse wave velocity in coal sample can be got. And the phase characteristics can be seen significantly in the process of wave velocity increase and finally the value of wave velocity remains stable. During loading, the longitudinal and transverse wave ratio decreases. The relation between the stress and the longitudinal wave is established and it can be known from theoretical analysis that with the increase of stress， the porosity decreases and the contact area between particles increases, which causes the increase of velocity.