A β-mannanase producing strain CYS4 was isolated from factory waste liquid and its enzyme activity reached 11.0 U/mL in the culture liquid of shaking flask.CYS4 was identified as Bacillus subtilis based on morphological observation,physiological and biochemical experiments and phylogenetic analysis with 16S rDNA sequence.Enzymatic characterization showed that the optimum pH for the enzyme was 7.0,it was stable between pH 5.0 and pH 11.0,the enzyme activity is more than 80％,and the optimum temperature was 60 ℃,it was stable under 40 ℃,the enzyme was still more than 70％ after 3 hours.The enzyme activity was activated by Na+,Fe2+,Mn2+,Zn2+,Mg2+,but inhibited by Hg2+.