【Objective】 Betula alnoides is a native, fast-growing, broad-leaved tree species found in the tropical and subtropical regions of China, and it has a high economic value and ecological benefits. The availability and spatial distribution of nitrogen (N) and phosphorus (P) significantly affect plant growth and root development. By studying the effect of fertilization on biomass allocation and root morphology in B. alnoides clones, the optimum B. alnoides clone and fertilization treatments were screened to provide scientific guidance for cultivating B. alnoides. 【Method】 Four B. alnoides clones (A5, FB4, FB4⁺ and BY-1) were selected as research materials, and a completely randomized experimental design was conducted with nine treatments including three N levels (0, 200 and 400 mg/seedling) and three P levels (0, 70 and 140 mg/seedling) with three replications and 25 plants for each subplot. To prevent nutrient deficiency in seedlings, 332 mg/seedling potassium (K) was applied. P fertilizer was supplied to the medium as a basal fertilizer. The application of N and K fertilizers was initiated at 1 month after seedling transplantation and was conducted at intervals of 7 days for a total of 10 times. When the seedlings were harvested, the roots were separated and scanned to measure morphological indexes (root length, surface area, volume and mean diameter). Finally, whole plants were dried to determine the biomass of each part, and the biomass allocation and root morphology of B. alonoides clones under different fertilization treatments were compared. 【Result】 ① The above-ground, below-ground and total biomass were significantly affected by fertilization treatment and differed according to clones, but they were not influenced by the interaction between clones and fertilization treatment. Compared with the control treatment, the fertilization treatment significantly increased the above-ground, below-ground, and total biomass. Regardless of the clone, treatment 5 (200 mg/seedling N+70 mg/seedling P) resulted in the highest biomass among all treatments, which was 11.27%-460.49%, 7.14%-200.00% and 10.57%-390.09% higher than the other treatments for above ground, below ground, and total biomass, respectively. On further comparing different clones, FB4⁺ showed obvious growth advantages, with an above ground, below ground, and total biomass of 1.14-1.21, 1.22-1.31 and 1.15-1.22 times than those of other clones (A5,FB4 and BY-1), respectively. ② The root surface area, mean diameter and total length were significantly different among different clones and fertilization treatments. Except the total length of roots with a diameter of <2 mm, the interaction effect between clones and fertilization treatment resulted in significant differences in terms of root volume, surface area, mean diameter and total length. The fertilization treatment improved root morphology compared with the control treatment. In all clones, treatment 5 resulted in superior root morphological indexes than the other treatments, with values 0.77%-227.20%, 2.13%-152.63%, 0.60%-264.13% and 4.53%-214.04% higher than those of root surface area, mean diameter, volume and total length, respectively. On further comparing the total lengths of fine roots of different diameters, treatment 5 also produced the longest thin fine roots (0 < diameter ≤ 1 mm), thick fine roots (1 < diameter ≤ 2 mm), and coarse roots (diameter>2 mm) in B. alnoides seedlings. The clone FB4⁺ under treatment 5 exhibited the optimum root morphology among all combinations of clones and fertilization treatment. ③ There were significant positive correlations among the above ground, below ground and total biomass. Further, these three measures of biomass were positively correlated with root mean diameter, total length, surface area and volume. 【Conclusion】 Fertilization significantly increased biomass and promoted root growth ofB. alnoides seedlings, although there were differences among clones. Based on biomass and root morphological indexes, clone FB4⁺ and treatment 5 (200 mg/seedling N+70 mg/seedling P) exhibited the optimum growth performance for this tree species.