Based on the discussion of the balance between supply and demand of water resources in Beijing in recent 12 years, water allocation and simulation (WAS) model was used to conduct a dynamic simulation study of water resources in Beijing from 1980 to 2018. The transformation model of regional natural-social water cycle was analyzed, and the transformation mechanism of social water cycle and natural water cycle was discussed. The results show that from 2007 to 2018, the average annual water demand of both real and imaginary parts of Beijing rises. The average of annual blue water supply is 3.685 billion m3 and green water supply is 1.69 billion m3. The water demand gap shows a significant increase trend with a perennial average of 16.52 billion m3. The verification results of the established WAS model show that the simulated and measured error rates of the annual average and characteristic frequencies of 25%, 50% and 75% runoff volume in Beijing from 1982 to 2010 are 7.0%, 4.8%, 1.2% and 0, indicating a high model accuracy. The flow rate of each calculation unit under the natural water cycle increases from northeast to southwest, while the actual flow rate decreases under the natural and social water cycle.