Abstract: Pak-choi (Brassica campestris ssp. chinensis Makino) is a native vegetable of China, which is critical for vegetable consumption. Pak-choi cultivation is often under weak-light environments. Studying the effects of different forms of nitrogen fertilizer on the growth, nitrogen uptake and related enzymes under weak-light conditions benefits proper application of ammonium nitrate for the optimum production of pak-choi. In this experiment, low-light environment was imitated using black shade net covers with the light intensity at about 20% of natural light. Then the natural light condition was used as the control. A controlled hydroponics experiment was conducted to determine the effects of five NH₄⁺-N/NO₃^--N ratios (0/100, 25/75, 50/50, 75/25 and 100/0) on nitrogen metabolism and key enzymes glutamine synthetase (GS) and nitrate reductase (NR) activities of pak-choi under weak-light conditions. The indicators for nitrogen metabolism and key enzymes were determined after 0 d, 5 d, 10 d and 15 d of treatments. The results showed that under weak-light, plant fresh weight, leaf nitrogen content, and GS and NR activities were significantly lower than under the natural light. Appropriate NH₄⁺-N/NO₃^--N ratio promoted pak-choi growth, maintained high nitrogen absorption, and higher enzyme activity. The total fresh weight, total nitrogen content, and NR and GS activities were highest at NH₄⁺-N/NO₃^--N ratio of 25/75, and lowest at NH₄⁺-N/NO₃^--N ratio of 100/0. Sustained high levels of NR enhanced the absorption of NO₃^--N, which in turn improved the growth and quality of the plant. High GS activity promoted the growth of pak-choi under weak-light conditions. The results suggested that increasing the proportion of NH₄⁺-N reduced energy consumption during nitrogen absorption, which enhanced the plant to adapt to weak-light conditions. The best effect was achieved under NH₄⁺-N/NO₃^--N ratio of 25/75.