Water resources is one of the most important supports for the sustainable development of society and economy. The paper proposed matter-element mode1 and extended relation degree in evaluation of water resources carrying capacity (WRCC). The matter-element model of evaluation scheme and ideal scheme were established according to observed data, and then the schemes were evaluated by the extended relation degree. The relation degree is an important index to evaluate regional water resources carrying capacity. The bigger the relation degree is, the better the close degree with the ideal scheme is. The results showed that evaluation of WRCC based on extension theory is feasible and reasonable, and it is a new model for the comprehensive evaluation of WRCC.