Abstract: To investigate the effects of purified humic acid (PHA) on the growth and nitrogen metabolism of cucumber under different levels of nitrogen stress and to explore the mechanism of PHA’s alleviation on abiotic stress, we examined the effects of PHA on ‘Xintaimici’ cucumber growth and nitrogen metabolism under low nitrogen (1.0 mmol·L^-1 NO₃^-) stress and high nitrogen (101 mmol·L^-1 NO₃^-) stress in hydroponics compared with normal level (11 mmol·L^-1 NO₃^-). The results showed that the growth of cucumber seedlings was inhibited under nitrogen stress treatments. Plant height, stem diameter, leaf area and dry matter accumulation under nitrogen stress were lower than that at normal nitrogen level. Dry matter accumulation under the normal nitrogen level and low nitrogen stress were significantly increased by PHA, but there was no significant difference under high nitrogen stress. The absorption of NO₃^- in cucumber seedlings was affected by PHA, showing increased nitrate content in cucumber seedlings under low nitrogen stress and decreased under high nitrogen stress. PHA significantly decreased ammonium content in roots and leaves under low and high nitrogen stress. Compared with normal level of nitrogen (CK), NR, GS, GOGAT, GDH activity in roots and leaves and NiR activity in roots significantly decreased under low and high nitrogen stress. PHA increased the activities of NR, NiR, GS, GOGAT, and GDH to different extents. PHA increased amino acid and soluble protein content in cucumber seedling roots and leaves. In summary, PHA addition alleviated the inhibitory effect of nitrogen stress on the growth of cucumber seedlings.