Abstract: In order to study the distribution characteristics of free gas content in coal reservoirs, based on the understanding that free gas mainly exists in coal pores, the estimation method of free gas content is optimized. Based on the test data of coalbed methane arameters of 42 groups of coals with different ranks from lignite to anthracite in typical coal-bearing areas of China, the free gas content of different coal samples were calculated by using the optimized method of free gas content estimation, and compared with the results of desorption gas content. The results show that the ratio of free gas content to total gas content decreases with the increase of mean maximum reflectance of vitrinite is from 2.13% to 33.25%. The relationship between the content of free gas and the average maximum reflectance of vitrinite is "double peak", and the peak value of free gas content in medium-rank coal is higher than that in high-rank anthracite stage 3. The order of the "double peak" value of the desorbed gas content is just the opposite, and the relationship between total gas content and vitrinite average maximum reflectance is mainly controlled by desorbed gas content.