Abstract: Rape straw treated by compound microbial fermentation was analyzed by using infrared spectra analysis apparatus and scanning electron microscope. The effects of biological pretreatment on biogas production were studied by using rape straw. The results showed that the cellulose and lignin and hemi cellulose degradation rate of rape straw, which was fermented by compound microbial agent for 11 days, reached 37.4% and 28.9% and 29.9%, respectively. The structure of pretreated straw has a great change and some crack and cavity, observed by SEM, were found in rape straw pretreated by compound microbial fermentation. The results of biogas fermentation experiment, rape straw as raw materials, showed that gas production peak daily and total biogas yield have remarkable increase, which was 37.6% and 17.8%, respectively. So, degraded the cellulose and lignin and hemi cellulose in rape straw by compound microbial inoculants could promote converting straw biomass into biogas.