Abstract: A kind of fly ash-based coagulant combining the capabilities of both physical adsorption and chemical coagulation has been prepared by adding a small amount of blast furnace iron sludge and an appropriate amount of helping solvent (HS) to fly ash in coal-fired thermal power plants and then the resulting mixture being stirred and extracted with a dilute H₂SO₄ for 2 h at 90℃. Combined with polysilicate aluminum and ferric (PSAF), this coagulant was thus used to treat tanning wastewater which COD value is 1 500～2 000 mg/L. The removal rates of SS, COD, sulfide and total chromium were 95%, 93%, 92% and 88% respectively. The remarkable characteristics of this method are a higher rate of settlement, a smaller volume of sludge, and a lower cost of treatment. The coagulation mechanism of fly ash coagulant in the treatment of tanning wastewater has been studied.