According to water balance principle, the paper established medium and long-term forecast method of Yellow River irrigation water demand. It analyzed the fundamental of irrigation water demand forecast and reviewed the main forecast parameters such as crop water demand, effective precipitation, irrigation water efficiency of sub irrigation system, and Yellow River irrigation water demand separation from the irrigation district water requirement. Crop water demand forecast could apply Valiantzas equation; effective precipitation prediction could use U.S. Bureau of Reclamation recommended method; and irrigation water efficiency of sub irrigation system could be calculated by some factors such as distance between final canal irrigation control unit to lateral canal division and channel lining rate; Yellow River irrigation water demand separation could be computed with irrigation area defined by remote sensing technique and the paper's method. Through the above research, it can provide technical support to the Yellow River agricultural water demand forecastion.