Beijing has received more than 6 billion m³ of water from the South-to-North Water Diversion Project （S-N project），which has greatly improved Beijing’s water resources carrying capacity （WRCC）. This study uses a system dynamics （SD） model to quantitatively access the effect of S-N Project on Beijing’s WRCC from 2018 to 2038. Firstly，based the assessment of the status of water resources，the assessment results show that （1）Since the S-N project was completed five years ago，the supply capacity of water resources in 2019 has increased by 11.1% compared to 2014，and the proportion of S-N project in the water supply system has increased to 19%；（2）since 2015，the S-N project is partly substitution of groundwater exploitation in Beijing. The proportion of groundwater exploitation has been reduced from 55% to 37%. The groundwater exploitation coefficient a has reached the second inflection point，meaning the rate of reduction has accelerated；（3）It is the two important factors，ecological flow （e-flow） and the optimization of Beijing’s industrial structure where water consumption per 10，000 yuan’s GDP reducing from 17.58 m³ in 2014 to 11.78 m³ in 2019，that have improved Beijing’s e-flow use. Secondly，this study uses a system dynamics （SD） model to quantitatively assess the effect of S-N Project on Beijing’s WRCC. The simulation results from 2018 to 2038 show that under the condition of the water scarcity within 5%，Beijing’s WRCC can promote the population carrying capacity from 21.54 million to 22.35 million. Beijing’s WRCC will be still dominated by domestic water and ecological water，where e-flow will be 1.66 times of the e-flow’s current demand. The amount of water diverted by S-N Project has increased by 22% compared the current diversion. The increase in water supply is mainly used to improve the quality of life of the people and moderate population growth. Domestic water consumption has increased by 20%，and ecological water consumption has increased by 32%. The per capita water consumption of urban and rural residents will also be increased，increasing by about 16% domestic water consumption per capita. Finally，it is concluded that S-N Project has a significant promotion effect on Beijing’s WRCC.