Through studying the mechanism and characteristics of water resources carrying capacity under urbanization, the leading factors of water resources carrying capacity under urbanization was analyzed. The evaluation index system and evaluation model of regional water resources carrying capacity based on the four dimensions of quantity-quality-space-stream were constructed. The proposed evaluation model was solved by chaotic particle swarm genetic algorithm (CPSGA). Taking Nanjing city of Jiangsu Province as the research object, the water resources carrying capacity of Nanjing from 2015 to 2018 was evaluated. The results show that the comprehensive evaluation model is more reasonable to be applied to the evaluation of regional water resources carrying capacity under urbanization, and the CPSGA algorithm can effectively optimize the weight of indexes, which reduces the subjectivity of the traditional evaluation method in index weighting, making the evaluation results more scientific and reasonable.