[Objective] To study the variation of undergrowth vegetation and soil properties during the growing process of artificial Robinia pseudoacacia in loess hilly region is of high significance to understanding the ecological impact of the plantations on the restoration of grassland ecosystem in loess hilly region.[Methods] We investigated the undergrowth vegetation and soil properties in 5, 10, 15, and 20 year artificial R. pseudoacacia in loess hilly region of Shaanxi Province, Northwest China, and analyzed the variations in undergrowth vegetation and soil properties during the growing process of the R. pseudoacacia.[Results] With the development of the plantations, the shrub crown width, height, sprout number, and basal diameter all increased significantly(p<0.05), but decreased in the 20 a artificial R. pseudoacacia. The species number, coverage and height of undergrowth vegetation decreased significantly(p<0.05), and the density of undergrowth vegetation had no significant differences among different development stages of artificial R. pseudoacacia. The soil conductivity and bulk density first increased and then decreased, soil pH had no significant differences among different development stages of artificial R. pseudoacacia, but the soil total porosity decreased significantly(p<0.05). The soil nutrients increased significantly(p<0.05), while they deceased in 20 a artificial R. pseudoacacia, and the soil total phosphorus had no significant differences in different development stages. The soil total nutrients of artificial R. pseudoacacia in loess hilly region had an obvious phenomena of "surface accumulation", which ordered 15 a > 20 a > 10 a > 5 a in the same soil layer with local fluctuation. Correlation analysis showed that the soil nutrients and morphological characteristics of R. pseudoacacia were the main factors affecting the species and individual number as well as the coverage of undergrowth vegetation.[Conclusion] It was suggested that in loess hilly region, the growing process of artificial R. pseudoacacia plantation is beneficial to the improvement of soil conditions and the recovery of undergrowth vegetation, and the 15 a artificial R. pseudoacacia produced the best effect in improving soil nutrient content. This effect decreased in 20 a artificial R. pseudoacaci, and did not changed the vertical distribution characteristics of soil nutrients. What is more, the influence on the undergrowth vegetation lagged behind its influence on soil nutrients.