Water resources carrying capacity (WRCC) is a recently proposed management concept, which aims to support sustainable socio-economic development in a region or basin. However, the calculation of future WRCC is not well considered in most studies, because water resources and the socio-economic development mode for one area or city in the future are quite uncertain. This paper focused on the limits of traditional methods of WRCC and proposed a new concept, water resources design carrying capacity (WRDCC), which incorporated the concept of design. In WRDCC, the population size that the local water resources can support is calculated based on the balance of water supply and water consumption, under the design water supply and design socio-economic development mode. The WRDCC of Chengdu city in China is calculated. Results show that the WRDCC (population size) of Chengdu city in development modeI (II, III) will be 997 ×104 (770 × 104, 504 × 104) in 2020, and 934 × 104 (759 × 104, 462 × 104) in 2030. Comparing the actual population to the carrying population (WRDCC) in 2020 and 2030, a bigger gap will appear, which means there will be more and more pressure on the society-economic sustainable development.