Abstract: Using the ultrafiltration method to divide Shixian sodium lignosulfonate (LS) into five molecular weight range Fractions. The gel permeation chromatography and potentiometric titration testing shows the relative molecular weight of LS is increased with the Fraction, from 2360 to 19852; the sulfonic acid content of LS is decreased with the Fraction, which is decreased from 2.66 mmol / L to 0.98 mmol / L. Using five Fractions’ LS as dispersants to prepare 40% myclobutanil WPs and test their performances, the results shows that WP’s performances are best when using the LS of Fraction4 (30000-50000) as dispersant, the suspension rate before and after heat storage are 85.36% and 81.38%, the average particle size of WP suspension’s disperse phase are 6.30 μm and 9.19 μm respectively, the 2 h suspension sedimentation layer thickness and the stability index of WP(after accelerated aging) are 0.311 mm and 3.85.