Abstract: Acetic acid lignin(AAL) was fractionated by dissolution in aqueous solution of acetic acid with various mass fraction. Four fractions with different relative molecular weight, which were chosen from the fractionation products, were sulfomethylated to obtain products with different relative molecular weight and sulfonic group content. The sulfomethylated lignin(SML) was added into myclobutanil as a pesticide dispersant to get a 40% myclobutanil wettable powder(WP). The influence of relative molecular weight and sulfonic group content on the application performance of 40% myclobutanil wettable powder was investigated. The results indicated that the suspension rate of myclobutanil increased from 72.68% to 83.69% and the wetting time increased from 45 s to 62 s when the average molecular weight increased from 5 768 to 13 964. The suspension rate of myclobutanil increased from 73.56% to 84.86% and the wetting time decreased from 72 s to 57 s when the sulfonic group increased from 0.51 mmol/g to 1.78 mmol/g. The average particle size of the dispersed phase of a 40% myclobutanil WP decreased when the relative molecular weight and sulfonic group content increased. However, too high relative molecular weight resulted in a decrease of the suspension rate of myclobutanil.