Abstract: Once antibiotics enter the environment, there will be a series of biological transformation process to eliminate them, such as hydrolysis, photolysis, biodegradation. In the laboratory, iron nanoparticles are proved better to degrade metronidazole. Numerous experimental studies have improved the efficiency of metronidazole degradation, few theoretical studies focused on the mechanism. This work investigated the degradation mechanism of metronidazole with density functional theory (DFT). Experimental results suggest a stepwise process catalyzed by Fe nano particles, finally the reactant nitro was catalyzed to amino through a series of reactions. Each intermediate product was simulated, the two oxygen atoms of nitro group are gradually replaced by hydrogens. The solvent effect is also considered. Calculation results show that the irons provide electrons, thus increasing the activity of the system, and the solvent promoted the reaction.