Abstract: In order to compare the different condition affected to the gas adsorption constant, the paper had a study on the gas adsorption experiments with five different coal rank samples under seven different pressure points. The experiments could be divided into constant volume and constant pressure dynamic gas adsorption experiments. The data of the adsorption equilibrium points under the conditions of the constant volume and constant pressure were obtained individually and in combined with the Langmuir single molecular layer theory, the adsorption constants a and b under the condition of the constant volume and constant pressure were obtained. A further discussion was conducted on the causes to make the differences of a and b values. The results showed that the two experiment conditions of the constant volume and constant pressure would have certain influences to the gas saturated adsorption volume (constant a), would have high influences to the gas adsorption rate (constant b) and the gas adsorption rate under the condition of the constant pressure would be higher than the gas adsorption rate under the condition of the constant volume. Further the adsorption constant a would be reduced with the volatile matter increased and thus the gas saturated adsorption volume would be increased with the coal rank increased.