Alachlor is used widely as a herbicide,but is an environmental endocrine disruptor. O3/H2O2 systemis used as catalyst to delve on the degradation efficiency of alachlor. The amount of the catalyst-H2O2 ,the pHvalue of the soluble, the temperature and quality of water sample are changed to investigate the effect of thesefactors on the degradation of alaehlor. The degradation of alachlor is qualitatively analyzed through their GS-MSspectra and the possible mechanism of the degradation of alachlor is discussed as well.