A highly active cellulose-decomposing strain XJ-25 was isolated from the soil in the wheat and maize straw returned fields using enrichment culture and Congo red staining test. The isolated strain was identified as Phaeosphaeria sp. XJ-25 by its morphological characteristics as well as by phylogenetic relationships based on 18S-ITS-5.8S ribosomal gene sequences. Conditions for liquid fermentation, a process that produces cellulase were optimized by the response surface methodology (RSM). The results showed that the conditions for CMCase activity reaching the maximum were: the corn stalk powder, 1.34 g·L^-1 (bran was 0.16 g·L^-1）; urea, 10.06 g·L^-1; initial pH of culture medium, 3.25; inoculation quantity, 4.13%; and CMCase activity reached 23.871 U·mL^-1. The conditions of FPase activity reaching the maximum were: the corn stalk powder, 1.34 g·L^-1 (bran was of 0.16 g·L^-1); urea, 10.12 g·L^-1; initial pH of culture medium, 3.41; inoculation quantity, was 4.22%; and FPase activity reached 8.653 U·mL^-1. The optimal reaction temperature of the crude enzyme solution was 50℃. When the temperature was over 40℃, the thermal stability of enzyme solution was poor, and the enzyme activity significantly decreased. The optimum reaction pH was 5. In the range of pH 4-6, the enzyme activity was more stable.