A thermomagnetic sliding bush stop sealing device for a nuclear reactor coolant pump comprises a sliding bush base and a sliding bush. The sliding bush base and a shaft are in interference fit, the sliding bush and the shaft are in clearance fit, the sliding bush is sleeved in the sliding bush base, a first seal ring, a second seal ring and a third seal ring are arranged on the sliding bush which is made of metal materials, and the sliding bush base is made of magnetic materials. The sliding bush is internally provided with two symmetric spring cavities, springs are arranged in the spring cavities, and the sliding bush base is provided with a hole. The sliding bush base and the sliding bush are attracted to compress the springs while the hole is sealed by the sliding bush. When a primary seal of the nuclear reactor coolant pump fails, high-temperature process fluid makes the sliding bush base lose magnetism, magnetic force between the sliding bush and the sliding bush base disappears, the sliding bush moves to the primary seal until being tightly pressed on the wall of a pump cavity under the action of spring force and pressure of the high-temperature fluid in the pump cavity, and the high-temperature process fluid in the pump cavity is prevented from leaking outwards. The thermomagnetic sliding bush stop sealing device is passive and free of external gas supply.