In present paper, based on the study of Xinhe paleo-ostracean reefs in the littoral plain of the southern part of Laizhou Wan (the Gulf of Bohai), we consider that the ostracean reefs are typical of a peculiar palaeoecological environment and are a good criteria which may be used both to identify paleo-coastline and to reconstruct palaeogeographic environment. According to the indicator of paleo-ostracean reefs, the existence of the Xinhe paleo-coastline is confirmed. The Xinhe paleo-ostracean reefs existed at 5000 to 6000yrs. ago (C14 dating), i.e. the relatively resistant stage of transgression during the post-glacial period. The characteristic of this stage is the development of ostracean reefs, therefore here termed "the stage of ostracean reefs". The sea level, at that time, would stand 2 to 3 m higher than at present.