Abstract: Based on Penman-Monteith formula, a prediction model of irrigation water demand was established to study the variation of irrigation water demand in 11 calculated regions of the North Three Rivers Basin under three inflow frequencies of 95%, 75% and 50%, and four scenarios of no rainfall increased, 5% rainfall increased, 10% rainfall increased and 15% rainfall increased, which is to explore the difference of irrigation water under different inflow conditions and rainfall conditions. The results show that the cloud water resources of the North Three Rivers Basin are rich and the precipitation efficiency is not high, which has great development potential. Crops need a lot of water from May to September. The planting structure and planting area of crops affect irrigation water demand. The irrigation water requirement of the North Three Rivers Basin is the highest in summer, followed by spring and autumn, and the lowest in winter. After analysis, we obtained that the combination of cloud water resources and farmland irrigation and optimizing crops planting structure can bring considerable benefits. During the dry year and extra dry year, summer is suitable for increasing rainfall in the whole basin of the North Three Rivers. Spring and autumn are suitable for increasing rainfall in Chaobaihe mountain area, Jiyunhe mountain area, Tangshan plain area, and Tianjin plain area, etc. The utilization of cloud water resources provides a new source of irrigation water and a new idea for the optimal allocation of water resources.