For many years the ecological stress caused by animal husbandry has been very high in the river resource regions, beyond the realistic carrying capacity of the grasslands, especially in Hainan prefecture. Using an ecological stress index defined as the ratio of realistic carrying capacity and theoretical carrying capacity of grassland, ecological stress indices of the Guoluo, Hainan and Yushu prefecture were calculated from 1961 to 2002. Evolution of the grassland ecosystem over the period was more seriously influenced by human factors than by natural factors such as temperature and precipitation changes. Numbers of sheep in the Hainan prefecture were much higher than the realistic carrying capacity. There appeared to be a time lag before the influence of human factors became apparent in the evolution of a river source region, which was related to the intensity of humanity activities, carrying capacity and the “anti-interference” ability of the grassland ecosystem.