Abstract: Systematic research on the porosity data obtained under different confining pressures from more than 30 coal samples is conducted considering the production characteristic of coalbed methane. Results show features of coal porosity varying with the confining pressures.And then, underground coal porosity calculation method and variational characteristic of coal effective pore volume compressibility are discussed.Moreover, an analysis of the difference between coalseam and sandstone as a reservoir is also included in this paper.