To prepare a highly efficient heterogeneous carbon-based magnetic catalyst with excellent solid-liquid separation properties and good stability for activating potassium peroxymonosulfate (PMS), this study employed Mn.Zn.FeO (MZF)magnetic nanoparticles (MNPs) as the magnetic core, and dopamine (DA) along with powdered activated carbon (PAC) was utilized to synthesize the magnetic nanocomposite MZF@PDA-PAC through a step-by-step deposition method. MZF@PDA-PAC was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and a vibrating sample magnetometer (VSM). The efficiency of MZF@PDA-PAC in activating PMS for the degradation of diclofenac sodium (DS) was investigated, along with the removal mechanism of DS and the activation mechanism of PMS by MZF@PDA-PAC. The results showed that MZF@PDA-PAC possessed a "core-shell" structure, which exhibited excellent dispersibility and solid-liquid separation performance in water. Both radical pathways (, HO· and ) and non-radical pathways (electron transfer) played important roles in DS removal in the MZF@PDA-PAC+PMS system, with DS and TOC removal efficiencies of 99.50% and 66.32%, respectively. MZF@PDA-PAC was shown to have high stability and good recyclability, which has broad application prospect in the degradation of refractory organic compounds.