Coal seam gas content is one of key indicators in marking CBM reservoir features. To precisely obtain gas content of low-rank coal seam, this paper, based on a case study on Dafosi coal mine, Binchang district, uses Langmuir formula to calculate the gas content of No.4 coal seam through measuring the critical desorption pressure during extraction, with the results showing gas content 2.30-3.62 m^3/t, 2.87 m^3/t on average, 0.8 to 1.93 times test gas content, corresponding to the feature of low-rank coal, very close to the original gas content of No.4 coal seam. This paper also discusses the reason of gas gush during mining No.4 coal seam according to rock movement theory and flow theory. Release of stress produces new fissures as top and bottom transform, collapse and move in the process of coal mining, changing coal seam structures and permeability, leading to pressure-relief-permeability-rising and flow-increasing effects, which makes gas gush of Dafosi coal mine.