Abstract: The plant biomass in mountainous grassland varies with the change of seasons: 1. The quantity of aboveground biomass is very lower in spring, grows rapidly in summer and autumn, and reduces in winter. Therefore, grazing would not begin too early in spring, or ceases too late in winter. 2. The quantities of underground biomass are larger than those of aboveground biomass in all seasons of the year. The total underground blomass varies in positive correlation with the change of biomass in 0—10cm soil layers. 3. The total blomass reduces during the rapid growth period of plant in summer and increases in autumn and winter. In the subtropical mountainous grassland ecosystem, the total biomass functions to supply forage for livestocks, to maintain the renewal of forage plants, to protect soil from erosion and to stabilize the grassland environments. Over-grazing must be avoided in the mountainous grasslands.