The aim of the study was to investigate the optimal fermentation conditions for cellulase production through solid state fermentation of Trichoderma viride. The study was designed based on the basis of single factor analysis. An orthogonal test was conducted by chosen three parameters, including the ratio of wheat bran to straw powder, moisture content and initial pH value as effect factors with the Box-Benhnken method. The quadratic regression equation was established based on the yield of enzyme, and the optimal conditions were determined using response surface methodology. The activity of Trichoderma viride cellulase displayed significant differences under different conditions (P<0.05). The optimal fermentation medium obtained when 2% ammonium sulfate was added at the culture temperature of 29 ℃ was as follows: wheat bran to straw powder ratio of 1.37:1, water content of 250% (10 g dry basis), and the initial pH value of 6.04. The highest enzyme activity of 59.72 U·g^-1 (dry media) was obtained after 72 hours, which increased about 20% compared to that of the basal medium.