In order to optimize the fermentation process of constitutive Pichia pastoris X- 33 for producing xylanase 1(XYL1),the fermentation kinetics during the single- tank chemostat cultivation was investigated,and the relation-ship between the specific cell growth rate and the cell productivity as well as the product yield was discussed.Then,based on the fermentation kinetics,batch control scheme was adopted to express XYL1 in a 50- L fermenta-tion tank.The results indicate that (1) the relationship between the specific cell growth rate and the glucose con-centration basically accords with the Monod equation,with a maximum specific growth rate value of 0.35h^-1 and asaturation coefficient value of 0.38g/L; (2) when the specific cell growth rate ranges from 0.024 to 0.212.the ki-netic model of XYL1 formation can be described using a quadratic equation with one unknown; (3) the specificsubstrate consumption rate is linear to the specific cell growth rate; (4) the optimal dilution rate is 0.316h^-1and the maximum volumetric cell productivity is 7.509g/(L·h); (5) the specific cell growth rate achieves 0.156h^-1when the product yield reaches its maximum; and (6) after fermentation,the wet weight of cells and the xylanaseactivity achieve (415 ±12)g/L and (2788 ±85)IU/mL,respectively,which means that XYL1 is successfully ex-pressed with high efficiency.