Abstract: Coal seam No.4 is one of the main mined seams in the study area. The research on the distribution of coal seam and its thickness change regularities as well as reasons provides the basis for coal resources exploration in next step and mine construction in the future. With the correlation analysis between coal seam thickness and other factors such as its overlying coal seam thickness, coal ash, sulfur and roof lithology, lithofacies etc., the paper investigated the reasons for the thickness change of coal seam in the study area and the surrounding area in combination with environmental analysis of coal seam No.4 during coal- forming period and coal-accumulating period. The results show that there is positive correlation between thickness of coal seam No.4 and its overlying coal measure thickness, coal ash and sulfur. The correlation coefficient is 0.810 1, 0.430 9 and 0.813 6. The coal seam No.4 was developed in the delta plain of shallow water delta, the thickness of coal seam was affected by syndepositional structures during the coal-accumulating period and river erosion after the formation of coal seam. Small changes of coal seam thickness is caused by the former factor, large changes of coal seam thickness caused by the latter, and even more seriously, there was non-coaliferous zone. In the process of peat accumulation, the coal seam thickness, ash and sulfur were reduced in the secondary uplift area; The coal seam becomes thicker in the secondary depressed area and coal ash, sulfur are higher.