Abstract: A thermophilic strain FL12 was isolated from soil samples, and its fermentation conditions of producing thermo-tolerant xyalnase were optimized by single-factor experiment and response surface analysis. The influences of medium composition and fermentation conditions on the fermentation were investigated. The optimal cultivation conditions were including carbon source was corncob at a concentration of 4. 9%, while the optimum nitrogen source was beef peptone (1. 0%) . A high xylanase activity of 2284. 06U/mL was obtained after 7d by incubation at 50℃ with initial pH6. 45, medium capacity of 60mL/250mL and rotation rate of 200r/min. The optimum temperature of FL12 crude fermentation broth was 75℃, 62. 94% residual enzyme activity was retained at 85℃.