Few study has been carried out on the desorption rules of extreme-soft and outburst-prone coal in south of China, especially coal from Hunan Province of which the value of “f” is normally less than 0.2. Desorption experiments are conducted on the coal samples from Hongwei and Jiahe Mines in Hunan Province, and the following phenomena can be observed: the calculating mean error by √t method and power function method of Hongwei coal samples are 29.68% and 68.33%, 28.55% and 70.28% of Jiahe , the calculating result by the two methods is always lower. After comparison we find that the √t method is more precise than power function method, so calculation method is improved based on √t method, mean error of gas content determination by the new method in experiment and coal mine are 6.88% and 5.24%, both less than 10%.