Abstract: From the structure of bank accumulations of Changhai Lake and lack of continuous upstream drift sheet in the end moraine bar, in view of the fact that there is no glaciation during the time when the dam is formed, as well as the formation of lake reservoir can’t be interpreted by “end moraine bar” theory and the huge age difference between ancient bank(53.6MAa) and young lake, this paper has denied the conclusion that Changhai Lake in Jiuzhaigou Resort Area, Sichuan Province is a glacier damming lake. Lithology of the bank accumulations and its lateral base rock are basically the same with vertical structure and with similar slope structure of other damming lakes caused by collapse, moreover, the regional geologic background is favorable for collapse and stable bank. It′s concluded that Changhai Lake is a damming lake of collapse cause.