To study the effect of different dissolved oxygen (DO) concentrations on the gill morphology and enzyme activity of a hypoxia-tolerant new strain F5 of Megalobrama amblycephala, we placed the new strain in fish tanks with low oxygen [DO: (1.7±0.2) mg/L] and high oxygen [DO: (19.3±0.5) mg/L] concentrations for 0 d, 4 d, 7 d, and recovery [DO: (7.8±0.3) mg/L]. The morphology of the gill tissue was observed from paraffin sections and malondialdehyde (MDA) content, catalase (CAT), succinate dehydrogenase (SDH), and lactate dehydrogenase (LDH) activities in the gills, hepatopancreas, intestines, and muscle were determined. The results of the paraffin sections showed that with longer hypoxia time, the interlaminar cell mass of the branchial filaments of the new strain decreased, whereas the surface area of the branchial filaments increased and recovered after 7 days of constant oxygen. Under hyperoxic conditions, the substratum intercellular mass increased and the surface area of the branchial patches decreased. The results of the enzyme activity test showed that the changes of CAT activity and MDA content in different tissues had no obvious regularity with the extension of treatment time under both hypoxic and hyperoxic conditions; however, there were significant differences (P<0.05). LDH activity increased significantly and SDH activity decreased significantly under hypoxic conditions (P<0.05), whereas LDH activity decreased and SDH increased significantly under hyperoxic conditions (P<0.05). This study provided baseline data on the effect of DO on the gill tissue and enzyme activity of different tissues and laid the foundation for the breeding of new strains of M. amblycephala.