Coal seam No.4 is one of the main mined seams in the study area. 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., this 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. 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.