Grassland desertification in Maqu County adversely affected the replenishment of the water sources in the upper reaches of the Yellow River. Based on an estimation of NPP and the theoretical livestock capacity of grassland, the effects of natural and artificial factors on grassland desertification were analyzed quantitatively. The results were as follows: the key natural factor that affected grassland desertification was increasing temperature, especially the accumulated temperature of ≥0 ℃, and the key artificial factor was overgrazing. The effect of temperature increase accounted for 34.39% of the grassland desertification and overgrazing for 65.61%. The grassland desertification phenomenon did not appear until the accumulative effect of overgrazing was considerable. An increase of 10 000 sheep units in the accumulated overstocking capacity would cause grassland desertification of an area of 3.14 hm2. Protection of grassland from desertification should mitigate or gradually decrease the accumulated numbers of overstocking on the basis of the forage livestock balance.