The predicted structure of Streptomyces olivaceoviridis xylanase XYNB was made by homology modeling and BLAST. Then N13D and S40E mutations were introduced into wide-type XYNB separately by site-directed mutagenesis to improve the enzyme thermostability. XYNB and the mutants (N13D、S40E) were expressed in Pichia pastoris and purified. Their enzymatic properties were determined. The result revealed that the thermostability of N13D and S40E were improved by 24.76% and 14.46% respectively compared with XYNB at 70℃ for 5 min. The specific activity of N13D was increased by 22% compared with XYNB. The other enzymatic properties of mutants were similar to XYNB. The mutants N13D and S40E are good materials for further research in the relationship between structure and function of xylanase XYNB.