In order to explore the effects of potassium fertilizer on the absorption and utilization of nitrogen fertilizer, and provide theoretical basis for scientific fertilization in apple orchard, Malus hupehensis seedlings were used to study the effects of different potassium levels (K0, K1, K2, K3, K4, K5, and K6 equivalent to 0, 2, 4, 6, 8, 10, and 12 mmol·L-1 K+, respectively) on growth and NO3- uptake and utilization using the track technology of 15N and non-damage microtest technique. The results showed that root vigor, nitrate reductase activity and root morphological index of seedlings in K3 treatment were significantly higher than those in the other treatments. The 15N derived from fertilizer (Ndff) in different organs of M. hupehensis seedlings was significantly different among treatments, and the Ndff values of root, stem and leaf appeared in K3 treatment were the highest, being 1.36, 1.33 and 1.47 times as those of the K0 treatment, respectively. With the increase of potassium application rate, the 15N utilization rate of seedlings were firstly increased and then decreased, and reached the highest value (23.3%) in K3 treatment, which was 3.04 times as that of K0 treatment. The results of non-damage microtest technique showed that there was a strong absorption of NO3- in K3 treatment (19.34 pmol·cm-2·s-1), while a clear trend of outflow was observed in the potassium deficiency (K0) and high potassium (K6) treatments. Therefore, the deficit or excess of potassium could inhibit the uptake and utilization of nitrogen, while appropriate potassium could promote root growth, enhance the activity of nitrate reductase, and then increase the absorption of nitrogen by seedlings.