A hydroponic experiment was carried out to investigate the effect of light intensity and NO₃^--N/NH₄⁺-N ratios on the content of total carbon and total nitrogen in different organs, root activity and nitrate absorption of Pakchoi. Results show that: ⑴when there was no ammonium in the nutrient solution, low light intensity (50% of nature light) decreased dry weight of the leaf, petiole and root by 73.8%, 82.2% and 74.4%; total carbon by 76.7%, 84.5% and 79.4%, and total nitrogen by 71.2%,79.5% and 80.4% respectively⑵low light intensity also decreased root activity and nitrate absorption of the plant significantly and ⑶under low light intensity, dry weight, total carbon, total nitrogen and root activity of the plant peaked when nitrate/ammonium ratio was 85 : 15 in the solution, and nitrate absorption by the plant decreased with increasing ammonium supply.