The conditions for cellulase production were studied with the mixed germs of Aspergillus niger (A.niger) and Trichoderma viride (R.viride) using straw stalk powder as carbon recourse by solid-state fermentation (SSF). The conditions of the mixed strains SSF were optimized by single factor experiments and orthogonal experiments in four aspects: the inoculation ratio, the bran-straw mass ratio, fermentation time and the filled volume. The optimal SSF conditions of the mixed strains were as follows: inoculation ratio of R.viride to A.niger was 1, the bran-straw mass ratio was 0.3, the time of fermentation was four days and the filled volume is 100 mL /1000 mL. The enzyme activity of FPA, CMC and β-glucosidase are 5.29 IU/mL, 9.33 IU/mL and 49.91 IU/mL in the optimal cultivation condition, which were 2.28~2.47 times, 2.39~2.45 times, 1.38~2.09 times as high as single-germ fermentation, respectively. The enzyme activity of cellulose from the mixed fermentation was higher than that from the fermentation of single bacteria.