Abstract: The connection between rivers and lakes is an important measure to improve the aquatic environment, restore freshwater ecology and improve the living environment of human beings. However, the quantitative evaluation of the dynamic connectivity of the lake water system connectivity project in the city is few. Based on the plan of Bajiao-South Lake connecting project in Yueyang City of Human Province, the water from the Yangtze River to Bajiao Lake will be self-flowing to the South Lake through a newly excavated river channel in the Wangjiahe River Basin to improve water quality and aquatic ecological functions of Wangjiahe River and South Lake. The MIKE21 hydrodynamic model was used to simulate the 12 operating conditions at the minimum control water levels of 25.86 m and 26.06 m at the South Lake exit under the flow rates of 6 m³/s, 12 m³/s, and 24 m³/s in flood season and non-flood season, respectively. The connectivity of the two lakes under water diversion conditions was analyzed and a quantitative evaluation index was proposed. The results showed that when the initial flow was 24 m ³/s and the minimum water level was 25.86 m, the connectivity between the flood season and the non-flood season reached the best state. The water exchange period was shortened to 5.59 d and 5.17 d, respectively. Judging from the changes in the connectivity index, the “leading to the lake” and the “two lakes connected” promoted water system connectivity and improved the ability of the water body to circulate.