The root system of birch in the Genhe area of the Greater Hinggan Mountains region of Inner Mongolia was taken as the research object.The analytical method of wood and all mining method were used to measure different old birch root biomass to study the distribution features of white birch root biomass.The results show that ratio of root and stem of birch changes between 0.1449 ±0.0439 and 0.0930±0.0090,and has a tendency to decrease with increasing age.For each age group of birch root system small roots account for small proporation,about 2% to 5%,medium roots occupied 12% to 26%,large roots and coarse roots were main contribution part of the biomass,making up 60% to 80%.With the increase of tree age,the big root biomass proportion decreases continuously,and coarse root biomass proportion enhances unceasingly,this might be the result of roots system periclinal growth.Based on the data of 20 parse trees,birch root biomass regression model,birch root biomass accumulated growth model,birch root biomass current annual growth model was established,each model has a high precision,which can be used in the Greater Hinggan Mountains Region of Inner Mongolia.