Under the background of the Grain for Green Project (GGP) and Cultivated Land Protection, the development process of cultivate land is vital to land spatial planning and ecological security of the forest-grass ecotone. Based on the spatial resolution data of 30 m land use in 1990, 2000, 2010 and 2018, we analyzed the spatial-temporal change of cultivated land in forest-grass ecotone by land use dynamic degree and transition matrix. Then we established the spatially explicit processes of cultivated landscape change process model, and divided expansion of cultivated land into bridge, branch, infilling and outlying, while divided the reduction of cultivated land into perforation, subdivision, shrinkage and attrition components. The results show that:1) the dynamic degrees of cultivated land in 1990-2000, 2000-2010 and 2010-2018 were 8.65%, -0.33% and -0.07% respectively, and the cultivated land increased by 2974.28 km² during the entire study period. 2) From 1990 to 2018, 2589.23 km² of grassland was transferred to cultivated land, which is the main source of cultivated land increase; The conversion from cultivated land to grassland was the main approach of cultivated land decrease before 2010, while in the later period, the occupation of construction land played a dominant role in cultivated land reduction. 3) The overall temporal expansion of cultivated land had a "outlying-bridge/branch-branch" tendency. The tendency was characterized by an early outlying type dominance, which was unordered, followed by its gradual disappearance. Furthermore, branch type relying on the original patch growth increased gradually, and a new dominance established accordingly. The whole process conformed to the law of the transformation of cultivated land from point or patch-type development to large-scale continuous development. The reduction of cultivated land was dominated by shrinkage in the early period, followed by attrition, and subdivision in the last period. Subdivision components increased gradually, the local government should pay attention to the fragmentation of cultivated land caused by construction land.