The lack of measured data on belowground biomass seriously affects the accurate estimation of grassland belowground carbon pool. By taking stipa klemenzii steppe as the research object, six sample plots were selected, and 2,160 root samples were collected for further experimental analysis via wild plots investigation, the seasonal dynamics of belowground biomass from May to September was analyzed. The results show that (1) the change trend of belowground biomass appears a "N" type, and the peak value appears in late July, and the lowest value appears in late August, the belowground biomass of grassland type of Caragana stenophylla Pojark-Stipa klemenzii+Cleistogenes songorica in each month is higher than that of grassland type of Stipa klemenzii+Cleistogenes songorica. (2) The seasonal dynamics of belowground biomass in different soil layers of Stipa klemenzii steppe is different, belowground biomass in the layer of 0-10cm appears a large fluctuation, which is consistent with the trend of total belowground biomass. Curve below the layer of 10cm is steadily stable. (3) There is no obvious correlation between below-biomass and soil organic carbon content in the soil layer of 0 -60cm. However, in the layer of 0-10cm, the soil organic carbon content is significant positive correlated with belowground biomass from May to July. (4) There is no obvious correlation between belowground biomass and monthly precipitation or monthly average temperature. (5) Belowground biomass in different soil layers has a significant positive relation with soil moisture; total belowground biomass is greatly influenced by soil moisture in the layer of 0-10cm.