[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.