An optimized process for carrying out solid-state fermentation (SSF) on pilot-scale by Aspergillus sulphureus was proposed on the basis of balancing the medium temperature and its water activity. The fermentation was carried out in wooden trays. The production was approximately 200kg of dry koji (solid-state medium) per day. The tests showed that xylanase production was greatly affected by the temperature and water activity of the medium. The suitable temperature for xylanase biosynthesis was in the range of 30-35 degreesC and the suitable water activity of the medium was in the range of 93-96%, which meant that the moisture content of the medium was in the range of 40-50%. During peak growth of A. sulphureus, a lot of heat was generated. Turning over the fermentation medium and ventilating the culture room were two effective ways to release the metabolic heat and maintain the medium temperature. However, these activities reduced water activity of the medium at the same time. Increasing air humidity was a good way to maintain the water activity of the medium. The xylanase activity of the dry koji was more than 1000 IU/g (dry koji) when the medium temperature and water activity were balanced. However, the xylanase activity of the dry koji was only 650 IU/g (dry koji) if the SSF was carried out naturally.