Dongting Lake, the second largest freshwater lake in China, is facing various challenges coming from relatively low water levels due to climate changes, the operation of the Three Gorges project, and the development of social economies. The lake functions are supported by certain hydrological conditions. This paper analyzes the river-lake patterns of this lake with different water stages at the Chenglingji hydrological station. These patterns are divided into four phases, i.e. river-like patterns at the water stages below 22 m, river-dominated patterns at the stages of 22-24 m, lake-dominated patterns at the stages of 24-26 m, and lake-like patterns at the stages above 26m, corresponding to lake-river indexes smaller than 1.0, 1.0-1.7, 1.7-4.3 and larger than 4.3 respectively. In-situ observations during 1950-2012 show that the annual mean percentage time periods during which the four phases occurred within a year, were 45%, 13%, 16% and 26% respectively. And the periods of both lake-dominated and lake-like patterns were presenting a decreasing trend. We have applied a wetted perimeter method to examine the relationship of lake functions versus water stage, and come to the conclusion: To maintain the functions of East Dongting Lake, the Chenglingji water stage should be in the range of 24-26 m and its annual occurrence longer than 60 days.