Abstract: The randomized block design was used to study the reaction mechanism of Acacia melanoxylon on different phosphorous fertilizer levels. Seven phosphorous fertilizer treatments (0, 50, 75, 100, 125, 150, 200 mg P/plant) were set up to measure and analyze the index of seedling height, ground diameter, biomass and root morphology. The results showed that all treatments had promoting effects on seedling growth, ground diameter, aboveground and underground biomass and root growth of Acacia melanoxylon. With the increase of time and phosphorous application amount, the root length, average root diameter, root surface area, root volume, seedling height, ground diameter and biomass of Acacia melanoxylon increased. The treatment of 100 mg P/plant had the best promoting effect, the root length, surface area, average root diameter, root volume, seedling height, ground diameter and aboveground and underground biomass of Acacia melanoxylon were 417.10 cm, 35.17 cm2, 0.47 cm, 0.48 cm3, 36.2 mm, 2.30 mm, 1.236 g (aboveground) and 0.227 g (underground), respectively. Meanwhile, the root length, average root diameter, root surface area and root volume increased by 81.2%, 110.7%, 61.1% and 127.2% compared with that of CK.