The study compares the spatial distribution characteristics of root systems based on different absorption criteria to clarify the impact of root classification criteria on the absorbing root distribution pattern. This study aims to improve the accuracy of root studies, and to clarify the effective "foraging" regions of tree roots. In a 7-year-old Populus tomentosa plantation, 780 soil columns were excavated around five sample trees. Root morphological indexes including root mean diameter (RD), root surface area density (RAD), root length density (RLD), and root volume density (RVD) were studied to observe their vertical and lateral distribution dynamics. The effects of different root absorption criteria on both the spatial distribution of fine roots and each morphological index of Populus tomentosa were also analysed. Results showed that choosing 2 mm as the criterion for absorbing roots would indeed lead to the transport roots being misclassified as absorbing roots, but their spatial distribution characteristics could still reflect the true spatial distribution pattern of absorbing roots. Moreover, under this criterion, the influence of the criterion on the actual shape and spatial distribution of fine roots was different due to the differences in monitoring index and study location. The spatial distribution characteristics of RAD, RLD, and RVD were basically the same, but the difference ratio of RVD was much higher than other indexes, and the difference ratio of the deep soil layer was generally higher than that of the shallow soil layer. Therefore, selecting RLD and RAD at the 2 mm criterion could more accurately reflect the true spatial distribution pattern of absorbing roots, and this standard was more suitable for conducting research in relatively shallow soil layers. When 2 mm was used as the threshold for studying the vertical distribution characteristics of fine roots by dividing the absorbing roots, it is recommended to describe the proportion of each morphological index in each soil layer. The current study was limited to Populus tomentosa, and the generalizability of the results across diverse tree species should be confirmed in future investigations.