Abstract: Chinese people began to research the main source of the Changjiang River 2400 years ago.Limited by the scientific level,they did not find out it. The Tuotuo River was determined as the main source of the Changjiang River in the 1970s.However,it was not correct,because when comparing the length of the Tuotuo River with the Dam River,the glacier length at headwater was added to the Tuotuo River,resulting in that the Tuotuo River is 1km longer than the Dam River.But glacier can not be regarded as river. In summer of 1986,we made a scientific investigation on the source of the Changjiang River and accurately measured the length of both the Tuotuo and Dam rivers on the maps.We discovered that the Dam River was 6.8km longer than the Tuotuo River,the Dam River 353.1km long and the Tuotuc River 346.3km.The discharge of the Dam River is 2.6 times as large as the Tuotuo River,The discharge of the Dam River is 196.18m³/sec,and the Tuotuo River 75.10m³/sec.The drainage area of the Dam River is 1.8 times as large as the Tuotuo River.The drainage area of the Dam River is 30715.7km²,the Tuotuo River 16691.0 km². Through the synthetic analysis of the factors mentioned above,we come to the conclusion that the main source of the Changjiang River is the Dam River instead of the Tuotuo River. We have corrected the position of the Tuotuo River head which Was not correct because of adding glacier to the length of the river,and also corrected the position of the Dam River head according to Landsat images.