Fighting and cannibalism in crabs are key factors that limit their production and economic potential. Fighting between crabs is not only affected by biological factors, such as gender and size, but also closely associated with the environmental conditions in which they live. As a common environmental factor, dissolved oxygen significantly affects the physiological metabolism of animals, subsequently changing the fighting behavior of individuals. To study changes in fighting behavior and energy metabolism of the swimming crab (Portunus trituberculatus) under different dissolved oxygen conditions, we used an observation system for fighting behavior that was constructed in our laboratory. The fighting behavior of swimming crabs was quantified under different concentrations of dissolved oxygen [(2.5±0.5) mg/L, (4.5±0.5) mg/L, and (6.5±0.5) mg/L]. Changes in glycogen in the hepatopancreas, as well as changes in glucose and lactic acid in hemolymph, were measured. The main results revealed as following: (1) With a decrease in dissolved oxygen concentrations, the fighting duration of the swimming crabs was significantly reduced, with the minimum value observed in the 2.5 mg/L treatment group, which was significantly lower than that in the other treatment groups (P＜0.05). Meanwhile, the fighting intensity between individuals also continuously decreased. (2) The frequency of fighting incidents for the swimming crabs significantly decreased with the decrease in dissolved oxygen, and the frequency of contact behavior and non-contact behavior among the winners in the three treatment groups were significantly higher than those of the losers in the fighting process (P＜0.05). (3) With the decrease in dissolved oxygen concentration, the levels of glucose and lactic acid in the hemolymph of the swimming crabs were significantly increased, and the level of liver glycogen was significantly decreased (P＜0.05). (4) After fighting, the liver glycogen level of the swimming crabs significantly decreased, whereas within the hemolymph, the glucose level significantly increased and the lactic acid level was significantly higher than what it was before fights (P＜0.05). The experimental results showed that the intensity of anaerobic respiration, energy supply efficiency, and lactic acid accumulation of swimming crabs were increased under hypoxia stress, which resulted in the decrease in fighting willingness and fighting intensity of the swimming crabs. Therefore, differences in energy metabolism may be one of the physiological mechanisms leading to differences in fighting behaviors under stress.