This experiment was conducted to study the mutagenesis breeding and optimal fermentation condition optimization of high yield fiber decomposition enzyme of Aspergillus niger. High cellulose decomposition enzyme of mutant strain was obtained through the mutagenesis breeding (ultraviolet, diethyl sulfate and ultraviolet-diethyl sulfate compound mutagenesis) of Aspergillus niger X1, and the fermentation conditions of enzyme production and the hydrolysis ability of straw were studied. The results showed that a mutant strain of Aspergillus niger X1U4-1 was selected through ultraviolet, diethyl sulfate and ultraviolet-diethyl sulfate compound mutagenesis of the original strain of Aspergillus niger X1; compared with the original strain, the filter paper enzyme activity increased by 66.7%, the β-glucosidase activity increased by 22.3%, the xylanase activity increased by 3.8% and the reducing sugar content (12 h) increased by 6.8% in mutant strain. The optimization of fermentation condition of Aspergillus niger X1U4-1 was that fermentation time was 84 h, corn kernel:bran=3:7, inoculation quantity was 1.2 mL, ammonium sulphate concentration was 4%; in this condition, the filter paper enzyme activity reached 0.77 U/g, cellulase activity reached 93.80 U/g, xylanase activity reached 9 383.18 U/g. This study shows that the mutagenesis of ultraviolet-diethyl sulfuric acid is effective in mutagenesis breeding of Aspergillus niger, thus to improve enzyme producing ability and straw hydrolysis capacity.