Abstract: In order to investigate the relationship between gas content and gas pressure, the Langmuir equation was deduced, the influence factors of adsorption constant was analyzed and summarized, and the internal relation between gas content and gas pressure was illustrated preliminarily. Meanwhile the relevant experimental study and theoretical calculation of coal of different metamorphic grade were carried out. The results show that the adsorption time, the pressure point setting, the temperature and the moisture content are main influencing factors of the Langmuir adsorption constant. The calculation results about gas content and gas pressure by modified Langmuir equation are related to the metamorphic degree. The low gas content may correspond to a high gas pressure in low rank stage, while the situation is opposite in high rank stage. Gas pressure 0.74 MPa and gas content 8 m³/t is only approximately consistent in lean coal. It is suggested to consider intergrately the features of geological structures and development of the tectonic coal to formulate reasonable outburst prediction index aiming at mines with coal of different metamorphic degree on the basis of the measured gas pressure.