A large terminal lake Zhuyeze once existed in Shiyang River Drainage Basin in prehistoric period.Since Western Han Dynasty (about 2010 years ago),the landcover of this area has changed greatly because of human activities.The irrigated farmlands replaced the natural oases and increased gradually,while the water was increasingly used for agricultural purpose.Zhuyeze shrank into many small lakes that dried up gradually.In this article the area of the “close lake”under modern natural conditions is calculated following water budget principle by deciding water income and outcome,supposed that the water resource in the drainage basin was not utilized by human being.The area of the modern terminal lake under natural conditions should be about 580km².