Owing to their large surface area and high biological activity, fine roots are the main vegetative organs by which trees absorb water and nutrients, and provide material support for aboveground growth and the physiological metabolism of other underground root systems. The senescence of fine roots affects plant growth and development, stand productivity and the chemical cycle of forest ecosystems. As one of the main tree species for water and soil conservation in the Loess Plateau, Robinia pseudoacacia plays an important role in improving the ecological environment. For this reason, R. pseudoacacia plantations in the Loess Plateau were selected as the study site, and the hand-digging and intact clod methods were used to obtain root system samples to analyze differences in the physiological indicators (e.g., vigor, soluble sugar content, soluble protein content, and cell membrane permeability) of the fine root senescence for plantation stands of different ages[i.e., young forests (11a), middle-aged forests (22a) and mature forests (34a)]. These results provide a detailed description of the growth and senescence mechanisms of the fine roots of R. pseudoacacia. During the growing season, the vigor of fine roots with respect to forest age, in descending order, was as follows:young forest > mature forest > middle-aged forest. As forest age increased, the soluble sugar content and soluble protein content increased, while the cell membrane permeability decreased. Higher-order roots were associated with increased fine root vigor and soluble sugar content, but nonlinearly decreased soluble protein content and cell membrane permeability. Based on an ANOVA, the fine root vigor, soluble sugar content (except for second-order fine roots), soluble protein content (except for first-order fine roots), and cell membrane permeability exhibited significant differences with respect to stand age within root orders (P < 0.05). The differences in fine root vigor (except for middle-aged forests), soluble protein content (except for young forests), and cell membrane permeability in roots at identical stand ages exhibited significant differences with respect to root order (P < 0.05). The soluble sugar content differed significantly only in mature forests (P < 0.05). In summary, during the growing season, fine roots of the young forests are more prone to senescence than those of the middle-aged and mature forests. The sequence of fine root senescence in Robinia pseudoacacia begins with first-order fine roots and progresses to second-order and the third-order roots.