Abstract: In order to study the geochemical characteristics and the genesis of CBM of Sijiazhuang mine field in Yangquan mining area, the in-site desorption test of coal core samples from the main gas-producing seams(seams 8, 9 and 15) from Permo-Carboniferous Taiyuan Formation was carried out systematically, gas samples were collected to conduct component and carbon isotope analysis. The results indicated that the gas content of seam 8 was higher than that of seam 9, on the plan the gas content of seam 15 was uneven, in some parts there was almost no gas. During the whole desorption, the gas component content and the variation regularity of seams 8 and 9 were similar, CH₄ content increased firstly and then decreased, N₂ content showed the trend of decreasing firstly and then increasing, while the CH₄ content of seam 15 showed the trend of nearly linear decline and N₂ content showed the trend of linear increase. The CH₄ volume fraction of seam 9_up was 27.82%~76.12%,N₂ volume fraction was 21.49%~72.20%. The CH₄ volume fraction of seam 15 was 38.15%~89.41%, the N₂ volume fraction was 6.55%~61.82%. With the process of desorption, the carbon isotope δ¹³C₁ of seams 8 and 9 showed generally the trend of increase, the overall δ¹³C₁ of seam 15 appeared as three series of gradual increase. There was no correlation in the variation of δ¹³C(CH₄) and δ¹³C(CO₂)。The CBM in the study area was mainly thermogenetic gas without obvious biological action. In addition, for the low CH₄ content and high N₂ content in the CBM components, further study is needed.