Abstract: The study aims to explore the response mechanism of Acacia melanoxylon seedling to K fertilizers, and provide a theoretical basis for Acacia melanoxylon seedling cultivation. A randomized block design was used to set potassium fertilizer application. Treatments with seven different concentrations were carried out. The results showed that the best fertilization range was 150-200 mg/seedling, and the effect of both treatment K- 200 and K- 150 were significantly different from other treatments. At the end of the trial, the ground diameter and seedling height of treatment K- 150 were 2.07 mm and 35.4 cm, while the underground and above-ground biomass were 0.19 g and 1.14 g, which increased by 27%, 33%, 90% and 97% respectively compared with the control. The root length was 380.10 cm, which increased by 60.3% compared with the control. The trial indicated that suitable potassium fertilizer application could promote the growth of Acacia melanoxylon seedling. However, if the application amount was too high (K-300) or too low (K-50), it would even damage the plant’s root system and affect plant’s growth.