Restoration of vegetation can also play a role in reducing soil erosion and improving the soil environment in the Chinese Loess Region. The vegetation recovery process, however, is slow and complicated, and the succession of vegetation can occur over the extended period. The root system is sensitive to the soil environmental response as it directly contacts the soil and plays an important role in the succession process of vegetation communities. The study of root system behavior in different succession stages can reveal the associated changes in the root structure and functional characteristics of plant community roots. The structure and ecological function of the root system are called root behavior characteristics, which is important to study for vegetation restoration in the Loess area. In this study, soil and root samples of 4 different vegetation communities (Artemisia capillaris, A. sacrorum, Bothriochloa ischaemun, and Lespedeza davurica) were collected in 2015 in Wangmaogou watershed of Wuding River. Each vegetation communities was arranged in two plots, and then 3-5 quadrants were established in each kind of plot. The inner diameter of 9 cm root drilling was collected vertically down to 50 cm, all the roots were picked out of the soil sample inside the root drill, and the soil and root samples were finally separately placed into a bag, which was sealed and brought back to the laboratory. The root samples were washed with water and divided into four diameter grades:0 > D ≥ 0.5 mm,0.5 > D ≥ 1 mm,1 > D ≥ 2 mm,and 2 > D ≥ 5 mm. Then, the root samples were scanned with a scanner. Finally, the root system parameters of length, surface area, tips, and diameters were determined. In the study of root ecology, a complex problem is how to reflect the ecological function of the root system through the characteristics of the root structure, that is, the mechanism of interaction between the soil interface and the acquisition of resources based on the characteristics of the root system. However, the problems of various plant communities under different soil site conditions and climates are considerably different, which could reflect the underground ecosystem, and the related research is particularly complex and difficult. Therefore, we analyzed the structure and functional recovery, the correlation, and effect on soil organic carbon of the root system of the plant community of returning farmland using nonlinear ecological simulation technology in the Chinese Loess Region. The results showed that root length densities (RLDs), root biomass, and root diameter increased slowly with the development of succession. The RLD of different diameter classes was mainly affected by species richness and coverage, and the root system distribution in different successional stages was uneven. The average fractal dimension of roots decreased from 2.77 to 2.59, and the niche index increased from 3.75 to 9.37 (both P < 0.05) in the 2-21 years of succession development. The ecological function of the root system significantly affected the soil organic carbon, and there was a significant negative correlation between the fractal dimension of the roots and the niche index (P < 0.01). Therefore, the structural characteristics of the root system directly determined its comprehensive ecological function and, therefore, the more complex the structure, the stronger the functionality.