In this paper, the degradation products of methidathion in water under natural conditions was identified, and then the components of commercially available methidathion pesticides were analyzed, finally the detection method of methidathion and its degradation product in vegetables was established by using QuEChERS pretreatment method. The results showed that the degradation product of methidathion in water was methidathion-oxon. In addition to methidathion, the content of its degradation product was as high as 1.5 g/kg. The linearity of the detection method was good over the concentration range of 1 ng/mL to 100 ng/mL (R²≥0.99). The recoveries was 62.9%~97.2% and RSD was 1.6%~10.0% at the spiked levels of LOQ, three times LOQ and 10 times LOQ. The method was simple, rapid and sensitive for the determination of methidathion and its degradation product in vegetables.