The invention discloses a method for quickly determining the gas content of coal seam according to a desorption rate. The method comprises the following steps: a, on-site sampling; b, on-site desorption test; c, desorption rate determination; d, calculation of the content of gas available for desorption; e, calculation of the content of original gas in a coal sample. The method can be used for accurately and conveniently obtaining the original gas content of coal seam, avoids the gas content measurement error caused by estimation error of a desorption model, does not need residual amount measurement, is implemented with simple equipment and is more convenient. In the same geological unit, under the identical conditions of the coal seam sample damage and industrial analysis, the desorptionrate under adsorption equilibrium pressure p1 does not need to be measured repeatedly, and the measurement is more convenient.