Ternary CoMgAl-LDOs were synthesized by co-precipitation and employed in the activation of peroxymonosulfate (PMS) for atrazine (ATZ) removal. A series of catalysts with different Co: Mg: Al ratios were obtained and characterized by XRD, SEM, EDS and XPS. Several key parameters such as Co: Mg: Al ratios, catalyst dosage, PMS dosage, initial pH and initial ATZ concentration were optimized. The results show that Co1Mg2Al1-LDO/PMS system exhibited better degradation performance and the maximum ATZ removal (98.7%) and TOC removal (26.5%) were obtained under the optimal conditions (Co1Mg2Al1-LDO = 75 mg/L, PMS = 0.4 mM and ATZ concentration = 10 mg/L at initial pH = 6.3). To verify the excellent performance in the Co1Mg2Al1-LDO/PMS system, three control experiments (i.e., PMS alone system, Co1Mg2Al1-LDO alone system and Mg3Al1-LDO/PMS system) and reusability tests were carried out. Furthermore, a total of eight intermediates were detected by UPLC-QTOF-MS/MS and the degradation pathways of ATZ were proposed. Besides, quenching experiments demonstrated that SO4 center dot- acted as the dominant active radicals and the amount of SO4 center dot- was quantified indirectly by HPLC. Finally, a reasonable reaction mechanism of Co1Mg2Al1-LDO/PMS system for ATZ removal was put forward according to the comprehensive analysis.