Abstract: Hydroponic experiment was carried out on M9T337 seedlings using ¹⁵N and ¹³C isotope tracer technology to study the effects of different potassium supply levels (K₀, K₁, K₂, K₃ and K₄ were equivalent to 0, 3, 6, 9 and 12 mmol·L^-1, respectively) on the growth, photosynthetic characteristics and ¹⁵N and ¹³C absorption and utilization of M9T337 seedlings. The results showed that dry mass, root length, root surface area, number of tips and root activity of M9T337 seedlings under the K₂ level were significantly higher than those under other levels. The net photosynthetic rate (P_n) of leaves increased at low K⁺ concentration and then decreased with the increases of potassium supply level, and reached the maximum value at K₂ treatment (15.5 μmol CO₂·m^-2·s^-1). At the 30th day after treatment, the activities of nitrate reductase (NR) and carbon metabolism enzyme were highest in K₂ treatment, and lowest in K₀ treatment. With the increases of potassium application rate, the ¹³C accumulation of seedlings were first increased and then decreased, with the ¹³C distribution rate of each organ being the most balanced at K₂ treatment. There were significant differences in ¹⁵N uptake and utilization rate among treatments. ¹⁵N uptake and utilization rates of seedlings under K₂ treatment were the highest, which were 16.1 mg and 17.9%, respectively. Therefore, too low or too high potassium supply could inhibit seedling root growth and leaf photosynthesis, which was not conducive to carbon and nitrogen absorption. Appropriate potassium supply could improve root activity and net photosynthetic rate, enhance nitrate reductase (NR) and carbon metabolic enzyme activity, and promote carbon and nitrogen metabolism.