Acid orange 7 (AO7) is an azo dye widely used in the dyeing and direct printing industry. AO7 is an environmental pollutant because the cleavage of azo bonds produces aromatic amines, which are considered mutagenic and carcinogenic. Microbial degradation is one of the most effective methods to remove environmental pollutants. A bacterium strain L-15 was isolated from the wastewater treatment system of a dye manufacturer. This strain is capable of decolorizing AO7. The strain was identified as Flavobacterium mizutaii based on its morphological, physiological and biochemical characteristics, and the sequence of 16S rDNA. The AO7-degrading characteristics and the effects of culture condition on the degrading efficiency of the strain were investigated by shake-flask culturing. The optimal degradation condition of L-15 was 30 °C and pH 7.0. After culturing at 30 °C for 3 days with the initial AO7 concentration of 20 mg/L, the degradation rate of AO7 was 60.45%. The optimal salt concentration was lower than 2%.