Abstract: Dissolved oxygen was a key operational parameter during xylanase fermentation process. In order to optimize dissolved oxygen for xylanase production,the effect of dissolved oxygen change accordingly with agitation speed and ventilation rate was investigated in a 30 L automatic fermentor. On the basis,agita-tion speed and ventilation rate were adjusted to control the dissolved oxygen at different fermentation stages. The results showed that the optimized strategy was found that at the earlier fermentation period, the dissolved oxygen was automatically controlled,at logarithmic phase the dissolved oxygen was controlled to 15% and in fermentation anaphase the dissolved oxygen was controlled to 45%. Under this conditions,the xylanase activity reached 2406.175 U/m L,which was 50% higher than before.