Abstract: Alkali lignin of wheat-straw from black liquor was processed by ultrasonic wave and the activation effect of ultrasonic wave on the alkali lignin was discussed. Functional group of alkali lignin was measured by chemical methods, molecular weight was measured by GPC and chemical structure was analysed by 1H NMR. Results indicated that ultrasonic processing was very effective. At ultrasonic processing time 20 min, power 200 W, and mass ratio (0.2 mol/L NaOH:lignin solution) 100:1, the contents of phenolic hydroxyl, aliphatic hydroxyl, carboxyl and aldehyde of the alkali lignin are from 1.88 mmol/g to 2.54 mmol/g, from 1.99 mmol/g to 4.14 mmol/g, from 0.59 mmol/g to 0.29 mmol/g, and from 2.16 mmol/g to 2.68 mmol/g, respectively. From the spectrum of 1H NMR, G-lignin percentage (the ratio of area of G-lignin peak to sum of all peaks'area) is from 3.61% to 0. S-lignin percentage is from 0.77% to 0, OCH3 percentage is from 11.50% to 8.90%. The numerical-average molecular weight (Mn) is from 1 179 to 5 031, the weight-average molecular weight (Mw) is from 10 250 to 11 605, dispersiveness(Mw/Mn) is from 8.69 to 2.31.