Abstract: Based on the study of the characteristics of coal formation in epicontinental coal accunulation basin,the viewpoint that coal was formed under marine transgression is put forward by the authors and the mechanism thereof is also analyzed.Marine transgression resulted in the forming of swamp and then the peat swamp. And the transgression later on put the peat formed in advance in deep water environment to form coal in the end. The important stratigraphic horizons, such as coal seams, marine facies layers, formed with marine transgression are of the property of isochronism.