[Objection] The objective of this study is to explore the ecological water demand calculation method in the southwest wet regions in order to provide scientific data for water resources allocation in karst areas. [Methods] We took Yanjin river basin in Guizhou Province as a case study. The Penman method, Kristensen-Jensen model, Mike SHE distributed hydrological model(DHI), and environmental function setting method was employed to calculate the optimal water demand from farmland, forest and grass ecosystems, water ecosystems in Yanjin river. [Results] The total amount of ecological water demand in Yanjin river was 1.39×10⁷ m³ to 2.04×10⁷ m³. Agricultural water demand in the basin was the most in April, July and August. The forest grass system didn't show the phenomenon of water lacking. The differences in water amount between dry season and abundant water period in the river were great, and the amount of available water was 3.84×10⁷ m³ to 9.17×10⁷ m³. To ensure the balance of ecological water, the main time of water intake was from May to August. [Conclusion] Farm crop water demand need to be concerned when water was taken from the river. The crop types and planting area also need to be considered when agriculture water was reserved. Moreover, we need to follow the rules of rainfall and ecological water use, to ensure the production of ecological water all the year round.