Based on Langmuir isothermal adsorption equation, we carried out isothermal adsorption simulation experiments under high temperatures and high pressures for different coal ranks. The results show that when vitrinite reflectance Ro less than 3.0%, the curves of Langmuir isothermal equation appear in a zone with the rising coal rank, temperature, and pressure. The adsorption capacity is enhanced with the rising coal ranks. The Langmuir volume of different coal ranks was less affected when the temperature was lower than 55℃, while more for temperatures above 55℃. The Langmuir volume increased obviously with the rising pressures when the low rank coal under 12 MPa and mid-high rank coal below 15 MPa. We established a mathematical model for determining gas content of deep coalbed based on the measured gas content extrapolation method and the high temperature and high pressure isothermal adsorption experiments. The modeled gas content shows variable outcomes, i.e., fast increase, slow increase, no increase, and slow decrease with the buried depth. The critical depth of low rank coal is located between 1400 m to 1700 m while the mid-high rank coal is located between 1500 m and 1800 m. The mathematical model of gas content provides a basis for forecasting gas content and evaluating deep coalbed methane resources.