Studying on gas desorption law about measuring coal seam gas content by compressed air sampling and exploring calculation method of losing gas quantity contribute to improving the accuracy of measurement result. At first, the gas desorption experimental device was designed and manufactured which can simulate measuring coal seam gas content by compressed air sampling. Afterwards, optimum experimental parameters of coal size and driving air pressure and gas desorption law about measuring coal seam gas content by compressed air sampling in different conditions were studied by experiment. Finally, the mechanism that compressed air driving promotes gas desorption of coal was analyzed, and the correction method of losing gas quantity inferred value was presented, and field test and application were carried out. The results show that compressed air driving may influence on coal specimens containing gas from vibration effect, fragmentation effect and dilution effect, and it may promote gas desorption of coal. On the border of compressed air driving desorption early stage and follow-up static atmospheric pressure desorption, there is a clear inflection point in the rising curve of accumulated gas desorption quantity. The relative error is 24.2%-61.8% between losing gas quantity inferred value determining by gas desorption law of follow-up static atmospheric pressure and actual value of gas desorption quantity in the compressed air driving period early stage. The revised relative error descends to 1.5%-14.0%, and the measurement result of coal seam gas content is improved by 3.72%-6.86%.