Abstract Amorphous Co78Si8B14 alloy, as a new potential catalyst, has hardly been used in the degradation of organic pollutant activated by peroxymonosulfate (PMS). In the study, the degradation performance of Orange II aqueous solution by PMS activated by amorphous Co78Si8B14 alloy in a heterogeneous catalytic process was studied at ambient temperature. The effects of initial solution pH, amorphous Co78Si8B14 alloy dosage and PMS concentration on Orange II degradation were examined in batch experiments. The results show that decolorization was strongly influenced by initial solution pH, amorphous Co78Si8B14 alloy dosage, and PMS concentration. The decolorization rate increased with the increase in pH value from 3 to 6.45. In contrast, it decreased sharply when the pH value of the solution increased from 6.45 to 11. The decolorization rate increased as the amorphous Co78Si8B14 alloy dosage and PMS concentration increased. Finally, the potential mechanism of Orange II removal by amorphous Co78Si8B14 alloy/PMS system was proposed systematically by comparing with the Co2+/PMS system.