Abstract: To improve the water environment of Dianchi Lake, based on the hydrological characteristics of Dianchi Lake Basin that are less affected by human activities, the minimum ecological water demand of Dianchi Lake under approximately natural conditions were analyzed. The study adopted the water level corresponding to 90% of the monthly empirical frequency method over the years as the lowest ecological water level of the lake. Based on the water level area relationship curve of Dianchi Lake, the corresponding lake surface area of Dianchi Lake was 297.485 km². By converting water surface evaporation into lake surface evaporation, the net evaporation of the lake surface was 585.1 mm, and the minimum annual ecological water consumption of the lake area was 174 million m³. The ecological water demand of the lake was calculated using the method of calculating the ecological water demand in the river channel, and various methods such as Tennant method was used to calculate the ecological water demand of the outflow river channel. The calculation results showed that the Tennant method was more suitable for determining the minimum ecological flow rate of Dianchi Lake (72 million m³), and the minimum ecological water demand of Dianchi Lake was 246 million m³. The research results can provide references for water ecology, water environment protection, and water resource scheduling and management in the Dianchi Lake Basin.