The selfmade circulating nutrient solution system was used to set the circulating nutrient solution with a hanging root length of 0 cm (T₁), a static nutrient solution and an initial hanging root length of 0 cm (T₂), a circulating nutrient solution and a hanging root length of 2 cm (T₃). With these three treatments, we measured the dissolved oxygen, pH, EC value of nutrient solution, root and leaf morphological and physiological indicators, and root microstructure observation, to clarify the role of rhizosphere oxygen environment in hydroponic lettuce mechanism. The results showed that: (1) the rhizosphere oxygen environment of the lettuce was the best T₃ treatment, the T₂ treatment was the second, and the T₁ treatment was the worst. The pH of the nutrient solution and the total amount of ions were not much different. (2) The growth of roots and leaves of each treatment showed T₃>T₂>T₁; the root length, root surface area, root volume, average diameter, number of root tips, leaf surface area and leaf volume, and plant stem diameter and plant height, dry weight of the roots, the dry weight of the shoots, the seedling index and the rootshoot ratio were the highest in T₃ treatment, but there was no significant difference between T₃ and T₂, and both were significantly larger than those of T₁ treatment. The roottocrown ratio was significantly increased by 38.8% and 13.4%, respectively, compared to T₁. (3) The area of vascular bundles and xylem of root lettuce showed T₃>T₂>T₁. (4) The activities of SOD and POD activities in T₁ treatment were higher than those in T₂ and T₃ treatments. The CAT activity and MDA content were significantly higher than that of T₃ treatment. The chlorophyll and four organic acids in T₁ treatment were lower than that of T₂ and T₃. The content of acid and succinic acid is the largest with T₂, and the content of malic acid and malonic acid is the largest with T₃. It was found that rhizosphere hypoxia stress significantly inhibited the growth, physiology and quality of hydroponic lettuce, and the oxygen in the air was more effective than the oxygen in the nutrient solution. Setting the appropriate length of the roots of the lettuce can increase the contact area between the roots and the air, thereby increasing the supply of rhizosphere oxygen, solving the watergas contradiction, and improving the yield and quality of lettuce.