The alkaline xylanase SX25 is produced by submerged fermentation of Bacillus subtilis followed by purification and formulation, and contains endo-1,4-beta-D-xylanase activity. It works together with chemical bleaching enzyme for whitening of pulp or paper. The product can effectively hydrolyze xylan in between cellulose fibrils into soluble smaller molecular products, loosening the fiber structure and improving permeability of cellulosic materials. Alkaline xylanase SX25 can be used for enzymatic treatment of straw, wood, bamboo, Chinese herbs, tobacco and feed.