In this paper, a slight halophilic alkaliphile strain, Alkalibacterium sp. F26, which produced high level of intracellular CAT observed in previous research, was selected as a model microbial to explain the responses of this bacterium to oxidative stress. The results indicated that Alkalibacterium sp. F26 had obvious responses to higher concentration (>1 mmol/L) of H2O2 than that to lower H2O2 (<1 mmol/L) challenge from the aspects of defensive enzyme synthesis and cofactors level variation. As for catalase production, the activity increased up to 106.54 U/mg protein which was 1.76 fold of the control when cells were challenged by 3 mmol/L H2O2, but its activity only was 1.13 fold when H2O2 was 100 mmol/L. As far as energy state was concerned, ATP production and NAD+ generation were significantly inhibited from 20.55 mmol/L to 17.80 mmol/L and 69.89 mmol/L to 31.77 mmol/L, respectively, leading to the drop of energy charge from 0.77 to 0.68 and the increase of the portion of NADH/NAD+ from 0.08 to 0.41 in the former case. However, these effects were less distinct under lower concentration of H2O2. Except of the condition of 100 mmol/L H2O2, under which the activation of defensive mechanism resulted in an increase of ATP, the level of ATP dropped from 22.69 mmol/L of the control to 22.38 mmol/L and 13.70 mmol/L when challenged by 50 mmol/L and 500 mmol/L H2O2. Besides, the concentration of NADH fluctuated and the NAD+ gradually reduced when H2O2 below 1 mmol/L.