Abstract: In order to understand the impact of grazing time on under-ground biomass of plant communities in desert steppe,variance analysis and regression modeling were used to systematically assess the under-ground biomass in desert steppe.The results showed that grazing time（early,mid-term and late stage）,root layer（0-100 cm） and the interactions between grazing time and root layer had very significant impact on the under-ground biomass of plant communities,and the highest amount of average under-ground biomass was observed in mid-term stage grazing in all root layers;the first root layer had the highest amount of under-ground biomass in all stages of grazing time,and the under-ground biomass was reduced with the increase of root layer depth.The fitting predictive model established in this study showed that the fitting degree of paddocks in early,mid-term and late stage grazing approached or reached 90%;mid-term grazing was beneficial to maintaining higher amount of underground biomass and was also favorable to the growth and development of the above-ground plant communities and to improve their drought resistance.