【Objective】This study aimed to investigate the root distribution characteristics and morphological differences in soapberry (Sapindus mukorossi) under different fertilization treatments. We also aimed to analyze the effects of nitrogen (N), phosphorus (P) and potassium (K) on the fine root growth and their interactions, so as to provide a support for the scientific cultivation of S. mukorossi, which is a widely planted biomass species in Southern China. 【Method】Using the eight-year-old forest in Jianning County, Sanming City in Fujian Province, as the research object, three levels of N, P and K fertilizers were set and a total of 14 treatments were conducted using the “3414” randomized block design. Among them, using no fertilization as the blank control (CK), three blocks were set up with 42 treatment plots and five trees per plot as repetition. At the end of the growing season in 2015, before the blooming period in 2016 and during the rapid growth period of fruits in 2016, fertilizer was applied in ditches, according to the specified proportions, in three rounds. In December 2016, four standard wood samples were selected from each treatment plot. Soil column samples were collected in three layers (0-20 cm, ≥20-40 cm and ≥40-60 cm) at a distance of 1 m from the trees. The distribution and morphology of the fine roots in the three soil layers of each treatment were studied. 【Result】The fine roots were mainly distributed within the 0-20 cm soil layer and gradually declined in the other two layers. The fine root biomass (FRB) and length density (FRLD) of the 0-20 cm layer were 1.51-2.52 times and 1.82-2.25 times higher, respectively, than the ≥20-40 cm soil layer, and 6.29-13.17 times and 6.03-9.31 times, respectively, compared to those of the ≥40-60 cm soil layer. FRB, FRLD, fine root surface area (FRSA), and fine root average diameter (FRAD) tended to increase initially and then decrease before becoming steady as the N, P and K fertilizer rates increased. Meanwhile, with the increase in fertilization, specific root length (SRL) decreased initially, before increasing sharply, and then decreasing steadily. Compared to CK, the FRB and FRLD of the N2P2K2 treatment clearly increased by 152% and 164%, respectively, in the 0-20 cm soil layer. Meanwhile, the FRB and FRLD also evidently increased by 242% and 161%, respectively, in the ≥20-40 cm soil layer, and significantly increased by 385% and 135%, respectively, in the ≥40-60 cm soil layer (P<0.05). 【Conclusion】 The FRB, FRLD and FRSA of S. mukorossi decreased gradually and had obvious vertical distribution characteristics in the 0-60 cm soil layer. Under the condition of P deficiency, N and K fertilizers were less effective on the root growth, and it was necessary to increase soil nutrient availa-bility to promote the fine root growth and biomass accumulation to some degree. When the amount of fertilizer was sufficient, this species could adjust the distribution structure of fine roots according to the availability of soil nutrient resources in the ≥40-60 cm layer and fully absorb nutrients. The interaction between N and P had a significant effect on the FRB, FRLD, FRAD and SRL, and was significantly correlated with the growth rate of fine roots. The fitting model of fine roots proposed that the most ideal amount of fertilization is N 693, P 321 and K 432 kg/hm².