Abstract: To address the functional decline of the natural spawning ground for Chinese sturgeon downstream of the Gezhouba dam and the water temperature constraints in the construction of an alternative experimental ground, this study selected a potential natural spawning site for Chinese sturgeon—the right-bank branch channel of the Yanzhiba reach in Yichang-as the research area, focusing on the problems of excessive water temperature drop and significant diurnal variation during the experimental period caused by poor water exchange in the branch channel. First, the suitable water temperature range for natural spawning of Chinese sturgeon was determined based on historical data. Second, through field monitoring and a coupled hydrodynamic-water temperature model, the causes of water temperature differences between the branch channel and the main Yangtze River channel under disconnected conditions were revealed. On this basis, four open-channel diversion schemes were designed to quantitatively evaluate the improvement effects of different diversion schemes on water temperature in the branch channel. The results show that under disconnected conditions during the experimental period (November–December), the water temperature in the branch channel is dominated by air temperature changes, being 2.0—5.0 °Clower than that in the main channel with a rapid cooling rate, leading to a significant shortening of the suitable water temperature window for natural spawning of Chinese sturgeon. Open-channel diversion can effectively reshape the water temperature field of the branch channel, synchronizing the water temperature within a certain distance downstream of the open channel with the thermal rhythm of the main channel, thereby delaying the time when the water temperature drops to the lower spawning limit from mid-November to late December, and extending the suitable experimental window to about 40 days. The effects of different open-channel scales on water temperature improvement in the branch channel were analyzed, demonstrating the economic viability and feasibility of a water temperature regulation strategy based on a small-scale excavation volume. Based on the recommended scheme, a natural spawning experiment of Chinese sturgeon was carried out in the upstream section of the Yanzhiba branch channel in 2025 and achieved success, verifying the practical effect of open-channel diversion in maintaining water temperature. This study reveals for the first time the maintenance mechanism and improvement effect of open-channel diversion on water temperature in the Yanzhiba branch channel, providing scientific basis and key technical support for the site selection of natural spawning experimental grounds for Chinese sturgeon.