The south coastal plain of the Laizhou Bay is a wide coastal area which lies between the outlet of the Xiaoqinghe River and that of the Jiaolaihe River. In this area , the industry and agriculture has developed quickly in recent years and the intrusion of underground saline water is very serious. In historic times , there existed three lakes in the south coastal plain of the Laizhou Bay: Judian Lake , Qingshuibo Lake and Biehua Lake. But they have already disappeared. To study the causes of disappearance of these ancient lakes is important to restore the palaeogeographic environments of the south coastal plain of the Laizhou Bay and establish the development direction of the lake-trace areas. Based on the data of the historical geography, the analysis of stratigraphic section, the textual research of the place name and satellite photographic interpretation, the authors studied the cause of disappearance of the ancient lakes in the south coastal plain of the Laizhou Bay, and pointed out that the causes are the drying climate, the migration of river and the activity of human beings. Among the three causes, the drying climate is the fundamental one. The migration of river is the direct cause and the activity of human beings speeds up the process of disappearance of ancient lakes.