Estimation modeling of the total biomass of shrubs is associated with certain difficulties. Moreover, merely studying the effect of organ allocation adaptability on shrub biomass is not sufficient. Thus, to investigate the effectiveness of biomass estimation modeling on organ allocation in plants, we focused on six common shrub species found in the forests of Tianshan Mountains, northwest China. To conduct a community investigation, we selected three sample areas in the eastern, central, and western regions of Tianshan Mountains. A number of whole standard plants from six shrub species were harvested to determine total biomass, which was divided into the, biomass of the roots, branches, and leaves, along with different classes of root diameter. D²H (the square of the diameter multiplied by height) and V (the area of the crown multiplied by height) were selected as the independent variables in the estimation models. Optimal estimation models for the total biomass of the six shrub species were constructed using regression analysis. These models were used to compare differences in total biomass allocation to vegetative organs and in biomass allocation to roots of each root diameter class in the shrub species. The following results were obtained. First, the total biomass of Berberis heteropoda, Lonicera hispida, and Cotoneaster melanocarpus was approximately 8.48 to 9.01 kg, whereas that of Rosa spinosissima, Spiraea hypericifolia, and Juniperus pseudosabina was approximately 2.71 to 3.20 kg. Second, the optimal estimation models for the total biomass of R. spinosissima, S. hypericifolia, and C. melanocarpus were functions based on V as an independent variable, whereas those for B. heteropoda, L. hispida, and J. pseudosabina were functions based on D²H as an independent variable. The value of R² for each estimation model was greater than 0.850, and all the models reached a high degree of accuracy at the 0.05 significance level. Third, differences in the proportional allocation of total biomass to roots and branches of the six shrub species were not significant, whereas that to leaves was significant at the 0.05 significance level. The root biomass of all six shrub species decreased with decreasing root diameter class, with significant differences in the proportional allocation of biomass to roots in roots with a diameter greater than 2 mm (significant at the 0.05 and 0.01 significance levels for roots with diameters greater than 20 mm). For all examined shrub species, we conclude that the differences in total biomass allocation to vegetative organs and in biomass allocation to the roots of each diameter class reflect adaptive strategies to their respective habitats.