Abstract: Ozonation was used to degrade residual BK809 in flotation wastewater and the degraded products in different stages were identified and quantified by gas chromatography-mass spectrometry(GC-MS).The results showed that the COD removal rate of 68.68% could achieve at an ozone flow rate of 300mg·h^-1 for 20min and pH 9-11.It was indicated from GC-MS analysis that BK809 was firstly oxidized to azobenzene and then converted to nitrobenzene.With the deepening of ozonation,the benzene was broken and further decomposed into C₁₅-C₂₅ carbon-chain alkanes and gaseous substances,including nitrogen oxides and carbon oxides.Finally the residual mass percentage of total organic compounds(TOC) reduced from 100.00% to 17.18%.Meantime,the chroma of the wastewater increased from 5.02 to 471.71 and then decreased to 28.75;in macroscopic view,the color of the wastewater turned from colorless to brown,and then from brown to milky.