Coexistence gain of populations has been studied for eight years， data from the field experiments of agroforestry system in Lixiahe, Jiangsu Province. Facilitation and resource sharing were identified as main contribution to the overyield of agroforestry. Resource sharing contributed 77.1％～81.4％ of biomass production to the total biomass of agroforestry in the threeyearold stands, decreasing with increasing treestandage. The contribution of facilitation to the total yield of agroforestry was lower than 3.3%, increasing with treestandage. The replacement of high lightrequired crops with low lightrequired ryegrass increased the gain of resource sharing and relative gain of resource sharing in agroforestry systems. The paper presented the tree thinning models for the maximum total biomass production of agroforestry and discussed the optimum control for the composition of tree stands in agroforestry systems.