Abstract: In order to improve the synthesis of β-mannanase by Trichoderma reesei,and determinate enzyme activity,fermentation medium of nutritional composition and proportion were investigated,which included carbon sources, carbon source concentration, nitrogen sources and ratio, carbon to nitrogen(m_C:m_N)ratio of four main factors on the biosynthesis of β-mannanase by Trichoderma reesei.The results showed that 20 g/L microcrystalline cellulose used as the carbon source, ammonium sulfate-urea mixture with the ratio of 1:1 used as the nitrogen source, the m_C:m_N=4 of culture medium composition were optimal nutrient for β-mannosidase synthesis of Trichoderma reesei.The highest β-mannanase activity and β-mannosidase activity were 4.48 and 0.04 μmol/(min·mL), respectively.The maximal β-mannanase activity was gained after 96 h fermentation.