Based on the hydrologic data from Chenglingji (Qilishan) Hydrologic Station, Dongting Lake from 1951 to 1988, the formula of hydraulic geometry in the section of Chenglingji-Dongting Lake mouth-are constructed according to L.B. Leopold's principle on hydraulic geometry of stream channels. in addition, the characteristics and changes of hydraulic geometry of stream channels are also studied. As a result, the stream channel width exponent b of Chenglingji outlet changes little with the discharge, whereas the deepth exponent f and the velocity exponent m change greatly with the discharge. Moreover, the cross section of the Chenglinji outlet is getting narrow and deep. The main reason is that the deposition processes of the Dongting Lake leads to an increase in the sediment transport capacity of Cherglinji outlet, while the eroding processes take place in the channel bottom mainly. in the past 40 years, because of the natural depositions of flood diversion in 3 waterways of the Jingjiang River as well as the influences of the curve cutoff of the Low Jingjiang River in 1967-1972, the values of m and m/f are increasing with each passing way and the sediment transport rate is decreasing obviously in Chenglingji outlet. All of these make sediment transport capacity increase and the eroding processes of the stream channel strong, resulting a deep and narrow cross section of the outlet.