Capacity of oasis systems is the result of mutual coupling by ecological capacity,eco-nomic capacity and social capacity.The mechanism of ecological-economic-social capacity inter-actions is driven by core factors of water,investment and population.The interior drive to realize the maximum ecological,economic and social benefit s of water factor decides the optimizing as-sembled ways of ecological-economic-social capacities.Under restriction of boundary benefit de-gression law,the optimum proportion of investment distribution lies in difference of unit invest-ment benefit.Population factor has double functions in the mechanism s of ecological-economic-social capacity interactions via changing the deficient degree of water resources and investment.