Abstract: Cultivated land is the primary resource for the sustainable development of agriculture, society and economy, and its protection has become one of the core objectives of land policy-making in China. Food production, ecological service and sightseeing are the basic functional demands in mega-urban regions, forming the regional particularity which should be fully analyzed in the threshold calculation of cultivated land protection. In a given time period, the demand degree for a certain function in a region depends on the supply sources of the function and its alternatives, and the factual function may not be consistent with the demands as a result of function’s imports from other regions. The total demand for grain has increased dramatically in mega-urban regions since more and more people have settled there. While sufficient food supply from domestic and international grain market has secured the great consumption, the factual grain production function of cultivated land has become weaker and weaker, but the potential grain production capacity of arable land should be a primary objective so as to face the unexpected challenge of food shortage. It is arable land, not only cultivated land, should be protected from this perspective. The demand for social security function of cultivated land is decreasing because more efficient rural social security system has been established in mega-urban regions. Demands for ecological functions, such as soil conservation, hydrologic modulation, air cleaning, and insulating construction land, have increased greatly as a result of serious ecological and environmental problems, while it cannot be imported from outer regions just as the grain consumption does, and also it cannot be supplied by “travel” just as sightseeing function does, it must depend on the region itself. So the ecological function should be an important concern in the calculation of cultivated land protection threshold. Furthermore, cultivated land should be planned systematically within the whole ecological land system. Based on above analysis, the minimum threshold model of arable land protection for food security (S_F), the minimum threshold model of tillage land protection for eco-security (S_E) and the total threshold model are developed on the base of minimum area per capita model and urban minimum ecological land use planning, such as ecological green equivalent, carbon-oxygen balance and landscape security pattern. The total threshold (S_A) is divided into tillage land (S_C) and other arable land (S_O), laying emphasis on potential productivity protection and structural optimization of ecological land. S_C is equal to the minimum value between S_E and S_B (the actual cultivated land of base year); if S_F is greater than S_C, S_O is the difference between S_F and S_C; if S_F is no larger than S_C, S_O is equal to 0. The total threshold (S_A) at the end of year 2030 is 104.17×10⁴ hm², with the tillage land (S_C) 48.16×10⁴ hm² and other arable land (S_O) 56.01×10⁴ hm². This case study indicates, to some extent, above calculation models are feasible and suitable.