Known organophosphorus pesticides are used widely in agriculture to improve the production of crops. Based on the literature, the degradation of some organophosphorus pesticides was studied theoretically. However, the mechanisms and variation of toxicity during the degradation of mevinphos and monocrotophos are still unclear in the environment, especially in wastewater. In this study, the reaction mechanisms for the degradation of the two representative organophosphorus pesticides (i.e., mevinphos and monocrotophos) in presence of OH radicals in the atmosphere and water are proposed using quantum chemical methods wB97-XD/6–311 + +G(3df,2pd)//wB97-XD/6–311 + +G(d,p). Result shows that the dominant channel is OH-addition to the C atom in CC bond with energy barriers being 15.6 and 14.7 kJ/mol, in the atmosphere and water, respectively, for mevinphos. As for monocrotophos, H-abstraction from NH group via barriers of 8.2 and 10.6 kJ/mol is more feasible in both the atmosphere and water. Moreover, the subsequent reactions of the major products in the atmosphere with NO and O₂ were also studied to evaluate the atmospheric chemistry of mevinphos and monocrotophos. Kinetically, the total rate constant is 2.68 × 10⁻⁹ and 3.86 × 10⁻⁸ cm³ molecule⁻¹·s⁻¹ for mevinphos and monocrotophos in the atmosphere and 4.91 × 10¹⁰ and 7.77 × 10¹¹ M⁻¹ s⁻¹ in the water at 298 K, thus the lifetime is estimated to be 36.46–364.60 s (2.53–25.31 s) in the atmosphere, and 1.41 × 10⁻² – 1.41 × 10⁻¹ s (8.92 ×10⁻⁴ – 8.92 ×10⁻³ s) in the advanced oxidation processes (AOPs) system. Furthermore, ecotoxic predictions for rats and three aqueous organisms imply their toxicity are reduced during degradation by using ECOSAR and T.E.S.T program based quantitative structure and activity relationship (QSAR) method.