Abstract: A gene encoding xylanase Xyn43 A from Aspergillus niger XZ- 3S was cloned into the Pichia pastoris expression vector,p PIC9 K. The recombinant plasmid was linearized with Sal I and then transformed into Pichia pastoris GS115 and KM71 by electroporation,respectively. After screening by MD medium,G418 concentration plate and shake bottle selection,two recombinant strains GS115 / Xyn43 A and KM71 / Xyn43 A were obtained. The best optimization schemes were obtained by single factor experiment and L₉( 3~4) experiment. The optimal expression conditions of GS115 / Xyn43 A were methanol concentration of 2.0%,inoculation time of 24 h,induction time of 108 h,induction temperature of 30 ℃,and initial p H6.3.The optimal expression conditions of KM71 / Xyn43 A were methanol concentration of 1.75%,inoculation time of of26 h,induction time of 132 h,induction temperature of30 ℃,and initial p H6.5. Under the optimum conditions,the xylanase activity of the two recombinant strains were139.36 U / mg and 143.29 U / mg,respectively.