The concentrated electricity demand of large-scale irrigation load causes a significant voltage drop in the power system, which severely affects the normal order of agricultural production. This paper first derives the mathematical model from irrigation electricity to crop yield. On the basis of this, a multi-time-scale economic operation approach for the combined agriculture and power (CAP) system considering large-scale irrigation load is proposed, which is composed of a day-ahead dispatch model serving for daily operation and a long-term planning model serving for the whole irrigation season. In the day-ahead irrigation dispatch model, economic dispatch model that meets long-term irrigation tasks is formed, which considers the optimization of key flexible resources such as reservoir and irrigation load, along with constraints of system security. In the long-term irrigation planning model, this study integrates affinity propagation clustering with the crop growth cycle demand, and formulates irrigation planning model based on typical scenarios, covering the entire production season of crops. Numerical simulation results reveal that the proposed approach improves the operation security and ensure the agricultural production of the combined agriculture and power system.