Abstract: The rule of aboveground biomass growth of the Stipa grandis steppe, shows a single peak form. This feature fits the climate of Central Asia with distinct cold and hot seasons, and the corresponding time of hot and rainy season. In the growing season, there are about 160–170 days for photosynthesis. The peak value of aboveground biomass and the peak growth hight appear at the same time, and this shows an obvious correlation(R = 0. 959). The highest growth rates of both appear 30–50 days ahead of their peaks. But their high relative growth rates are in early spring. Although the biomass is different from year to year, but in the composition of biomass, the proportion’s of grasses and forage are alway the same. After cutting in early spring (May), the reproductive biomass is at its height. Viewed from the sum of biomass and reproductive biomass at different times, the cutting midsummer (July) affects most exerts the the productivity of the steppe. The production of the plants under 40cm high makes up 85–90% of the total biomass.