Abstract: Coal seam gas content is a fundamental parameter in the evaluation and development of coalbed methane resources, prediction of coal seam gas protrusion risk as well as prevention and control of gas disasters. At present, long-distance and long-drilling confined sampling is widely used in gas area management. This study analyzed the desorption process and patterns of underground confined sampling gas in order to calculate the amount of gas loss during the sampling process. We proposed a novel method of loss calculation based on Sun Chongxu's formula, and developed an auxiliary software for calculating the amount of loss of gas by using the C# language and MySQL database. Results show that the average loss of gas sampled from confined sampling obtained by the new method and auxiliary software accounted for 13.15% of the gas content. The gas content measurement in the coal seam yielded results that were 4.04-6.58% higher than that of the traditional sampling method of 'catching powder at the mouth of the hole'. This method and the auxiliary calculation software could calculate the amount of gas loss from sampling, thus improving the accuracy of gas content determination, and meeting the engineering demand in long-distance and long-drillhole confined sampling.