Abstract: The Late Permian coal-accumulating area in eastern Ynnan and western Guizhou is an important coal-forming area in south China.The paper,based on geological data and practices,disscussed and sumed up the depositional characteristics and coal-forming regularities in tidal flats in the area.The results indicated that specific depositional structures of tidal flat system were abundant in the area,mainly included single clay layer,double clay layer,tidal cyclic sequences,bivariate rhythmic structures of up-tide and down-tide deposits,flat and round mud pebbles,bidirectional mud pebbles,gradational cyclic tidal sequence and mud scour channels,etc.Depositional sequence of tidal flats and planar sand bodies were evident.Sand bodies in tidal channels were generally parpenticular to the shore,and often pinched out towards the land.Wide and stable high quality coal seams were mostly formed in tidal flat environments under relatively stable tectonic conditions.But in areas such as tidal channels,lagoon and gulf flooded by sea water for long time,sulfure content often increased in coal seams.It was pointed out that tidal flat system was one of the important coal-forming environments for stable high quality coal seams in the area.