Abstract: To obtain xylanase with high activity and low cost, optimization of medium composition for the production of extracellular xylanase by Aspergillus niger SL-05 was investigated in solid-state fermentation using statistical experimental designs. The results of preliminary experiments showed that culture medium containing apple pomace and cottonseed meal (1︰1) as carbon and nitrogen sources supported xylanase production with maximal enzyme activity. Three significant factors, urea, KH2PO4, and initial moisture contents that influence xylanase production were selected using Plackett-Burman design. Further experiment was carried out using design of rotation-regression-orthogonal combination to study the influences of process variables on xylanase production. 2.6% Urea, 0.09% KH2PO4 and 62% initial moisture content obtained by the response surface plot were optimum for xylanase production. The analysis of variance for models shows that the tested model is statistically significant (P<0.05) and lack of fit is not significant, indicating that they are appropriate and can be used to predict the effects of urea, KH2PO4 and initial moisture contents on enzyme production. Under the optimized conditions as follows: ratio of apple pomace and cottonseed meal 1︰1, urea content 2.6%, KH2PO4 content 0.09%, glucose content 2%, initial moisture content 62.9%, fermented at 30℃ for 60 h, 5662 and 30000 U/g of xylanase and cellulase were produced, respectively. This study innovatively developed a fermentation medium and process to utilize inexpensive agro-industrial wastes to produce a high yield of xylanase.