Seasonal dynamic changes of the Stipa steppe community biomass after exclosure were studied in the paper. The results showed that the above-ground biomass cohered to a Logistic curve both inside and outside the exclosure, and had a remarkable relevance to soil water. The above-ground biomass inside exclosure is much higher than that of the outside exclosure. The under-ground biomass formed a typical reversed pyramid, special distribution was uneven, mainly concentrated in 0～20 cm under ground. The under ground biomass inside exclosure was also higher than that of the outside.