It is essential to clarify the resource list of unstable cultivated land for maintaining ecological balance, ensuring regional food security and promoting the sustainable development of economy and society in the arid areas of Northwest China. At present, the definition and classification of unstable cultivated land in China is relatively general, and the resource endowment of different regions is quite different, so it is difficult to highlight the characteristics of unstable cultivated land in the arid areas of Northwest China with a national standard classification system. In this paper, the connotation and concept of the unstable cultivated land in the arid areas of Northwest China were expounded, and the classification system was constructed according to the unstable factors. Taking Changji Prefecture as a typical case, this paper analyzed the quantity, quality and spatial distribution of unstable cultivated land. The main results are as follows. (1) The definition of unstable cultivated land in the arid areas of Northwest China is as follows: limited by ecological factors (desertification, soil salinization, etc.), or production factors (water, soil, etc.), or socio-economic factors (location, farmland infrastructure, etc.), it is difficult to stabilize production, and continuous cultivation will threaten ecological security of cultivated land ecosystem and surrounding environment, and there is uncertainty in the follow-up land use types. Unstable cultivated land can be divided into four types: productive unstable type, ecological unstable type, socio-economic unstable type and mixed unstable type. (2) The area of unstable cultivated land was 14.56 ten thousand hm2 in Changji Prefecture, accounting for 22.37% of the total cultivated land area. Ecological unstable type, productive unstable type, mixed unstable type, and socio-economic unstable type accounted for 36.54%, 34.03%, 17.94% and 11.49% of the total area of unstable cultivated land, respectively. (3) The unstable cultivated land was mainly at low-grade level in Changji Prefecture, and the fourth to sixth grade cultivated land accounted for 79.96% of the total area of unstable cultivated land. Our results are of great significance for the deep understanding of the unstable cultivated land in the arid areas of Northwest China, and provide scientific support for the effective management of cultivated land resources in the case area.