BACKGROUND: It is necessary to inoculate microbes in the composting of special materials. Biogas residue (BR) is rich in refractory lignocellulose. To improve humification processes of BR composting, composite microorganisms (CMs), with multiple functions, were added to the mixture of BR and animal manure. RESULTS: The results indicated that four humification indices (DP, PHA, HR, HI) were all markedly higher in the inoculated compost (0.46, 27.5, 43.34, 11.92) than in the uninoculated compost (0.39, 22.53, 28.56, 6.49) (P<0.05). The inoculation increased the degree of polycondensation and aromatization, and molecular size, as well as the humification degree of BR compost as determined by fluorescence spectroscopy and fluorescence regional integration analysis of humic acids, fulvic acids andhydrophilic fraction, three chemically isolated fractions. Additionally, the inoculation increased the quantity and diversity of indigenous microorganisms and accelerated their proliferation by adjusting environmental conditions to optimize their growth. CONCLUSION: The CMs inoculant is suitable for BR composting and inoculating based on the composition of the composting substrate could enhance composting efficiency and improve compost quality. (C) 2014 Society of Chemical Industry