Abstract: In this study, a series of tests were conducted to explore the exogenous RSI effects on anaerobic digestion of mesophilic waste-activated sludge (WAS) from the perspectives of hydrolysis and acidification efficiency, biogas production and surface morphological variation of digested sludge. The results showed that: (1) Acetic acid was the major hydrolytic-acidogenic intermediate product during WAS digestion. The concentration increased at low dosage but decreased at high level of RSI. (2) The medium addition of RSI (no more than 20 g/L) promoted the acetate and butyrate fermentation but inhibited the propionate fermentation. The anaerobic digestion efficiency of WAS was thereby improved. (3) The cumulative methane yield at RSI concentrations of 0, 1, 5, 10, 20 and 30 g/L were 135.4, 141.9, 159.2, 178.9, 209.3 and 180.7 mL/g, and the corresponding methane content was 51.2%-56.4%, 53.9%-58.6%, 58.1%-62.5%, 59.5%-68.3%, 61.1%-71.2% and 51.9%-61.4%, respectively. The optimal RSI dosage was 20 g/L. Compared with the blank group, the cumulative methane yield and methane content increased by 54.6% and 23.0%, respectively. (4) The SEM-EDX analysis showed that the microbial metabolism contributed to the dissolution of RSI. The iron content on the surface of digested sludge increased with the increment of RSI dosage. (5) The activities of protease and cellulose were promoted by moderate amount of RSI, but was suppressed by excessive RSI dose. Overall, the results demonstrated that the anaerobic digestion efficiency of WAS was improved by moderate RSI doses (no more than 20 g/L).