Coal bed methane is unconventional raw natural gas stored in coal seam with considerable reserves in China． In recent years，as the coal bed methane production，the safety and the use of resources have been paid more attentions． Evaluating coal bed methane content is an urgent problem． A BET adsorption isotherm equation is used to process the experimental data． The various parameters of BET equation under different temperatures are obtained; a theoretical gas content correction factor is proposed，and an evaluation method of actual coal bed methane is established．