Abstract: Abstract: Determination of theoretical carrying capacity is one of the most essential requirements for the evaluation of forage-livestock balance in pastoral areas of Northwest China. Degradation, desertification, and salinization of arid sandy grasslands are becoming increasingly prominent for the reason of unreasonable grazing in recent years. Intermittent grazing has been implemented for the protection of fragile grassland ecosystem, leading to an imbalance between the grassland supply and livestock demand. It is highly urgent to accurately evaluate the forage-livestock balance for pastoral areas in arid sandy grassland. Taking the pastoral areas in Otog Banner as the typical study area, this study deeply analyzed the natural matching between grassland supply and livestock demand and proposed two dynamic factors reflecting the annual fluctuation of stocking capacity and the changes in grassland productivity in a wet, normal, and dry year. On the bases of those the improved key pasture approach was built. Traditional, key pasture, and improved key pasture approaches were used to calculate the theoretical carrying capacities of pastoral areas under various rainfall conditions in different water resource regions of Otog Banner. Then the forage-livestock balance in pastoral areas was evaluated using the Improved Key Pasture approach combing with the Balance of Grassland and Livestock Index (BGLI). The results show that the natural processes of grassland supply and livestock demand were significantly imbalanced in time for pastoral areas of arid sandy grasslands. A dynamic cycle included 5 sub-processes using the natural matching in annual animal husbandry. Meanwhile, the Improved Key Pasture approach was more suitable for the calculation of theoretical carrying capacity in the pastoral areas in arid sandy grasslands, compared with the Traditional and Key Pasture approaches. The theoretical carrying capacities were achieved for warm season pastures, cold season pastures, and artificial grassland. There was a downward trend of theoretical carrying capacities with the decrease of rainfall. The proportion of cold season pastures was too high to be fully utilized in all three divisions. The theoretical carrying capacities of the whole year in the three divisions of Zhuozishan Mountain, Dusitu River Watershed and Inland River Watershed were 86.0, 534.9, and 276.6 thousand sheep units in the status quo year, respectively. In the evaluation of forage-livestock balance, there was the most serious overload for Inland River Watershed division in the status quo year. The BGLIs of each grassland and the whole year were about 105% in Inland River Watershed division, then followed by that in Zhuozishan Mountain division of about 45%. In Dusitu River Watershed division, the forage-livestock balance was relatively good, but still in marginal overload states. The finding can contribute to the protection and restoration of the grassland ecosystem for better development of the regional economy and society.