Coal gasification slag (CGS), a kind of solid waste generated during the production of coal gasification, was used as persulfate (PS) activator to effectively remove phenanthrene (PHE), a typical polycyclic aromatic hydrocarbon difficult to eliminate from aquatic environment. The microstructure and elemental composition of CGS were characterized by SEM-EDS and XPS. The types of free radicals were determined through free radical quenching experiments and EPR experiments. The effects of CGS and PS dosages, initial pH value and coexisting substances on PHE removal in the CGS/PS system were investigated, the oxidation mechanism was explored, and the degradation pathways were predicted. ECOSAR software was used to evaluate the toxicity of PHE and its products. The results showed that 11.9% of Fe was identified in CGS, indicating that CGS was an effective PS activator. The degradation efficiency of PHE reached 95.34% after 60 minutes of reaction, with the initial conditions of PHE dosages of 1 mg/L, pH value of 3, CGS dosages of 0.7g/L and PS dosages of 1mmol/L, and the utilization rate of PS reached 29.18%. The degradation process of PHE conformed to pseudo first-order reaction kinetics，and the sulfate radical (SO4?-) and hydroxyl radical（HO?）were produced. SO4?- was found to play a dominating role during the oxidation, and HCO3- and NO3- inhibited the reaction significantly. PHE was decomposed into long-chain acids and esters through decarbonylation, hydroxylation and decarbonization, and finally converted into CO2 and H2O. The ECOSAR model evaluation found that the toxicity of intermediate products is generally lower than that of PHE.