Abstract: The seam B9b, which was formed on a lower delta plain,was distinguished into five microlithofacies by light of coal petrology,and the microlithofacies derived respectively from five type of peat-forming microenvironments, i. e.,wet forest swamp Ⅰ、Ⅱ、Ⅲ、Ⅲ、Ⅳ and dry forest swamp. The microenviroments,in which the wet forest swamps were dominant,gone through three developed cycles (stages)from dry to wet in the peat-forming course of the seam.