An increasing interest is devoted to combined microbial electrolysis cell-anaerobic digestion (MEC-AD) system which could convert waste activated sludge into biogas. In this study series tests were initially conducted to study the effect of alkaline pretreatment on AD system and the results showed that alkaline pretreatment could promote the dissolution of organic matters in the sludge and thus improve the methane production. Then, the methane production in combined MEC-AD system fed with alkaline-pretreated sludge was investigated. The results indicated that the methane productions increased by 37% and 42% when applied voltage was 0.5 V and 0.8 V. The microbial electrochemical system strongly promoted the growth of Euryarchaeota (Methanosaeta and Methanobacterium). Meanwhile, the abundance of Paraclostridium increased from 17.9% to 38.5% when applied voltage was 0.8 V, suggesting an enhanced fermentation and acetogenesis process. The results of energy balance estimation indicated that MEC-AD system at 0.5 V could achieve higher net energy output.