There exists synchronous growth in resources consumption and environment pollution in Kunming urbanization in recent years，which has led to the increasing pressure of urban sustainable development． Considering the environmental and social benefits in Kunming's water resources sustainable development，the authors provided a research finding the optimal urban population scale in different stages for urban water supply based on the sustainable development groundwater evaluating theoretical model． Through the research on the availability of construction site as well as the risk in underground space exploitation with the extenics theory method，we get a suitable partition map of land for urban construction and a risky partition map of underground space development． The results show that the appropriate population scale of Kunming will be less than 2. 96 × 106 by 2020，and will reach ( 4. 34--2. 96) × 106 optimized population scale after 2020 under the condition of economic- socially and environmentally sustainable development． It is significant to optimize urban construction land and use underground space under the condition of land resources sustainability in Kunming．