Abstract: The decision support system of water requirement forecast for dryland crop is a systematic integrated intelligent computer software established by combining existing agronomy knowledge, model and expert experience with the artificial intelligent technology, on the foundation of Penman formula. This decision support system is a subsystem as “Expert system of water-saving agricultural of Northwestern area”. It can give decision consulting for irrigation scheme for the management of winter wheat and summer corn in Middle of Shaanxi areas. And can led the water-saving farming to more scientific and more reasonable in this areas.