The secondary drain flow regulating valve of the Moisture Separator Reheater System of a nuclear power plant is frequently blocked, which will reduce the efficiency of the recovery and utilization of the drain water generated by the secondary reheater, thus affecting the efficiency of the thermal cycle. By comparing the drain control valves of other systems in the plant and the operation and maintenance of peer power plants, the power plant has determined the reasons for valve blockage: the main component of valve blockage material is Fe3O4, and the main reason for Fe3O4 is the precipitation of iron in the medium after saturation. Based on this, the temporary mitigation measures for on-line flushing are formulated. At the same time, the valve with blockage problem is redesigned and reconstructed, and the structure of the valve cage is improved. The design of the new structure valve cage is not conducive to the accumulation of impurities, and the cleaning of the valve cage is easier. The power plant will use the modified new valve cage in unit 2. After the transformation, the drainage capacity can meet the needs of the unit and the regulation performance is good. After the unit is at full power, it is observed for four months, and it is confirmed that the anti blocking ability meets the requirements.