Abstract: We study the changes and relation of porosity and acoustic wave velocity during loading with the medium rank coal from Gujiao Mine Area of Xishan Coalfield. The research results show that with the increase of confining pressure, the coal samples porosity decrease exponentially. And the porosity in different directions has different stress sensitivity during confining pressure changing. With the increasing of confining pressure, the nonlinear increasing of the acoustic wave velocity in the coal samples can be got. And velocity anisotropy can be found in coal samples. On the basis of previous studies, the relationship between wave velocity and porosity is founded using confining pressure state as a bridge. It can provide a methodology to estimate approximately porosity from acoustic wave velocity to Gujiao Mine Area.