Simulated sunlight irradiation causing degradation of amidosulfuron, a pyrimidinylsulfonylurea herbicide, has been investigated in aqueous solution. The main degradation products were followed up by ultrahigh-pressure liquid chromatography with a UV detector (UHPLC-UV) and identified by combining ultrahight-pressure liquid chromatography-mass spectrometry (UHPLC-MS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). On the basis of the retrosynthetic analysis, the most identified degradation products were mainly due to the losses of methylsulfamic acid (CH5NO3S), sulfocarbamic acid (CH3NO5S), carbamic acid (CH3NO2), methyl(methylsulfonyl)sulfamic acid (C2H7NO5S2), N-methylmethanesulfonamide (C2H7NO2S), and sulfonic acid (H2SO4) molecules. Accordingly, O and S-demethylation as well as hydroxylation processes were also observed. Sum formulas of the main degradation products were assigned, and a mechanical pathway is proposed.