The invention discloses a reversed spitting device for a silage harvester. The device comprises a feeding part, a chopping device and a catapulting part, wherein a forward rotating drive belt and a reversed rotating drive belt are arranged between the drive wheel and chopping part of the chopping device, and are respectively provided with a tension pulley; and an action drive element of the tension pulley is connected with an electronic control mechanism. The action drive element of the tension pulley is controlled by the electronic control mechanism, so that the tension pulley can compress the forward rotating drive belt or the reversed rotating drive belt, and the chopping part is controlled to rotate forwardly or reversely to realize switchover between the normal cutting and reversed spitting of the chopping part. The device is convenient and rapid to operate, and has high efficiency.