The invention discloses a shutdown sealing device for a nuclear power main pump. The shutdown sealing device comprises a movable sealing ring installed on a shaft sleeve, and a static sealing ring installed on a mechanical sealing end cover. The sealing end face of the movable sealing ring and the sealing end face of the static sealing ring are oppositely installed. The shutdown sealing device further comprises a driving mechanism used for pushing the movable sealing ring towards the static sealing ring in the axial direction. The driving mechanism is composed of a driving outer thread located on the cylindrical face of the movable sealing ring, a non-rotating nut which is matched with the driving outer thread and is relatively fixed in axial position, and a control mechanism enabling the nut and the outer thread to be in a disengaged or engaged state. The shutdown sealing device for the nuclear power main pump is driven under residual inertia when a rotating shaft is shut down, rigid attachment sealing of the two sealing end faces is achieved, and the shutdown sealing device has the beneficial effects of being simple in structure, high in reliability, good in sealing effect, simple in control system and the like.