In this paper, a framework for conceiving an index that can quickly evaluate biogas conversion potential (BCP) of sewage sludge (SS) was proposed based on the surface site of sludge particulate (SP). As enzymes, the cornerstones of metabolism, are responsible for the efficiency of anaerobic digestion (AD) process, the adsorption of hydrolytic enzyme on the sludge with different surface site density (SSD) of SP was studied. The experimental results suggest that the surface site of SP can be considered as the binding site for hydrolytic enzyme, as confirmed by organic solubilisation tests of the sludge. To further explore the relationships between SSD and BCP of SS, the biochemical composition and long-term biochemical methane potential (BMP) tests of different sludge samples were conducted. The results show that both the methane production rate constant (k) and net cumulative methane production (NCMP) increased as the SSD values increased. Analyses of the statistical and fitting mathematical models for the SSD, biochemical composition, k, and NCMP revealed a significant positive linear correlation between k and NCMP versus SSD can be found (R-2 = 0.910, P < 0.001 and R-2 = 0.939, P < 0.001, respectively), indicating that the SSD of SP is a potential index to quickly evaluate the BCP of SS. These findings can enlighten environmental scientists on developing an evaluation index for the BCP of sludge without a long-term BMP test. (C) 2017 Elsevier B.V. All rights reserved.