Alkali lignin has low reaction activity because of its chemical structure, but its reactivity can be improved under microwave irradiation. The trimethyl quaternary ammonium salt was synthesized under microwave irradiation with wheatalkalilignin as raw material, which was extracted from black liquor from the kraft process. Effects of reaction temperature, reaction time and catalyst on the synthesis were investigated. The optimal synthesis conditions were determined: reaction temperature 75 ℃, reaction time 25 min, and catalyst was not necessary because the microwave irradiation played a similar role as a catalyst. The flocculation of trimethy quaternary ammonium salt prepared from whea-|alkali-lignin was examined by acid black ATT dye in an aqueous solution, and the results showed that over 90% acid black ATT dye solution could be flocculated when the trimethyl quaternary ammonium salt at a concentration of 600 mg/L with pH values of 1-2.5 was added into the acid black ATT dye with a concentration of 0.15 g/L.