Welan gum is a soluble exopolysaccharide produced by Sphingomonas sp. The fermentation conditions were optimized by single-factor experiments using a welan gum producing strain Sphingomonas sp. 511 screened in our laboratory. The optimal fermentation conditions were as follows: inoculum age 15 h, inoculum volumn 10% (V/V), medium volumetric ratio 16% (V/V), initial pH value 8.0, rotate speed 300 r/min, culture temperature 32 ℃ and fermentation period 72 h. The scale-up test was carried out under these conditions and the concentration of welan gum reached 25.76 g/L.