Method for measuring content of free gas in coal bed gas

The invention discloses a method for measuring the content of free gas in coal bed gas. By calculating the surface water saturation of a fresh coal core and performing back calculation to obtain water saturation in the stratum state, the volume of the free gas in a coal rock of unit mass is measured. Preferably, the method comprises the following steps of: collecting a sample, namely collecting a cylindrical coal sample which is not polluted by drilling fluid; measuring the volume Vtotal of the coal sample; measuring the volume Vwater1 of initial water; measuring the volume Vwater2 of saturated water; measuring an effective burden pressure delta p and a volume coefficient B; and according to parameter results in the preceding steps, calculating the content of the free gas. By adoption of the method, the content of the free gas can be accurately measured, so a total gas content test result is approximate to an underground true value, and great significance is provided for exploration and development of the coa