Abstract: In order to study the indicating effect of illite for coalification and sedimentary environments, the Jurassic coal seam gangue in Jingxi coalfield of North China was taken as the research object. X-ray diffraction(XRD) and X-ray fluorescence(XRF) were used to determine the mineral composition and chemical composition of the gangue sandwiched in the coal seam gangue. The mineralogical characteristics and genesis of illite in the gangue were studied. The results indicated that the intra-seam partings are mainly composed of illite, chlorite and quartz, followed by small amount of calcite and albite. The illite has an average proportion of 62.92%. According to the comparison of XRD data and XRF data, the average chemical formula of the illite can be calculated as (K_0.37(NH₄)_0.03)Al₂(Si_3.60Al_0.40O₁₀)(OH)₂. The illite in the intra-seam partings of Jurassic coal of the Jingxi coalfield is mainly converted from kaolinite, and the NH₄⁺ proportion of the illite has an increasing trend with the increase of depositional paleosalinity, and the n(Si)/n(Al)^Ⅳ of the illite has a decreasing trend along with coalification. This indicates that the mineralogical characteristics of the illite in the intra-seam partings can be used as indicators of coalification and depositional environment.