The invention discloses a rear side door assembly for a silent box body shell structure. A right vertical rib and a left vertical rib are both fixedly installed on the front end face of a rear door plate through core pulling rivets; a cross rib is fixedly installed on the front end face of the rear door plate through a core pulling rivet; a damping plate is fixedly installed in a groove of the rear door plate, and makes contact with the bottom face of the groove of the rear door plate; sound absorbing cotton is fixedly installed in the groove of the rear door plate and is ejected on the damping plate; a right positioning pin and a left positioning pin are both fixedly installed in installing holes in the bottom face of the rear door plate; a right water tank support is fixedly installed onthe front end face of the right vertical rib through a bolt, a nut, a gasket and a spring gasket; and a left water tank support is fixedly installed on the front end face of the left vertical rib through a bolt, a nut, a gasket and a spring gasket. The rear side door assembly for the silent box body shell structure is simple and compact in structure, simple in machining process, low in cost and good in sound insulating effect.