The invention relates to a bioelectrochemistry auxiliary anaerobic composting device capable of improving putrescibility of dewatered sludge and a startup operation method thereof, relating to an anaerobic composting device and aiming at solving the technical problems of slow composting progress, long period and poor composting maturity in the existing anaerobic composting methods. The composting device comprises an anaerobic composting chamber, an electrochemical electrode chamber and an electricity storage or consumption device, wherein the anaerobic composting chamber is positioned in the center of a reactor, formed by a space surrounded by an internal shell and provided with a first graphite brush electrode, and the side wall of the internal shell is a proton exchange membrane; and the electrochemical electrode chamber is formed by spaces between an outer shell and the internal shell of the reactor and provided with a second graphite brush electrode, and an electrode liquid is added in the electrochemical electrode chamber; and the first graphite brush electrode and the second graphite brush electrode are connected by a copper conducting wire to form a closed circuit. The composting device disclosed by the invention can compost dewatered sludge in 35-40 days with the composting maturity 28%-30% higher than that produced according to the existing composting methods, and is used for anaerobic composting.