Basic and extended coupled process models of an agricultural ecological-economic system were set up using system evolution theory and Thames′ coupled model of vegetation and soil erosion. The model revealed the interactive process of agricultural ecological-economics and showed the critical point at which the agricultural economic system impacted the ecological system. This has important practical significance for the utilization of agricultural resources and the development of sustainable programs for the agricultural ecological-economic system. Based on more than 20 years of system evolution information from the Zhifanggou Watershed of the Loess Plateau, key parameters (k1,k2,k3,k4) which revealed the direction and intensity of interaction between the agriculture-economic and ecological systems as well as adjustment parameters (α1,α2) which indicated the evolutionary track transformation of the economic and ecological systems resulting from significant behavioral processes were calculated. Then the coupling process model was established to reflect the coupling process between the agriculture-economic and ecological systems in different situations. The results showed that the evolutionary speed of the agricultural-ecological system was negatively related to the state of the economic system but positively related to the state of the ecological system. There was also a positive relationship between the evolutionary speed of the agricultural-economic system and the state of both the agricultural economic and ecological systems. The critical point of self-regulation for the current evolutionary track of the agricultural ecological-economic system in the Zhifanggou Watershed was en(t)=1.22×el(t). At this critical point, the evolutionary speed of the economic system was 0.051×el(t). which means that under the conditions of the self-balanced ecological system, the economic system reached its greatest evolutionary speed of 0.051×el(t). If the systemic circulation and the “forest and pasture resources-animal husbandry” chain network continue to be perfected and strengthened, then the economic system function could be enhanced within the framework of the threshold of the ecological system. According to the “Rotation Pasture and Barn Feeding” management systems, if 300 sheep were raised during the early days in the Zhifanggou Watershed under the potential grazing capacity of 500 sheep, then the calculated values would be 0.557 for α1 and 0 for α2. There was no significant effect on the critical point of self-regulation for the agricultural ecological-economic system, and the maximum allowable evolutionary speed of the economic system was 0.073×el(t). If the “Grain for Green Project ” were stopped and the reclamation was emerged, the function of economic and ecological systems would both fall. We hypothesized that if the “Grain for Green Project” were cancelled, the conditions for farmland in the Zhifanggou Watershed were slightly better than conditions in 1999, the calculated values would be 0.1 for α1 and -0.386 for α2. The critical point of the ecosystem self-adjustment changed to en(t)=0.69×el(t), the maximum allowable evolutionary rate of the economic system changed to 0.047×el(t), and the unit ecosystem function was weakened. To this end, we should stabilize the “Grain for Green Project”, promote the efficient utilization of agricultural resources, and upgrade industries in the region. Only in this way, can the agricultural ecological-economic system achieve healthy and sustainable development.