This paper develops the system of optimal allocation of water resources in irrigation district and irrigation decision-making support which integrates technologies of decision-making support, information management, information search and so on. It has the multi-function of water production function calculation, crop water requirement calculation, water resources optimal allocation and real-time amending the decision of irrigation. This system integrates the experience of experts with computer technology to guide farmers to irrigate in a proper way, for which the limited water resources can produce a marked effect on irrigation, so irrigation district management and efficiency are improved.