The utility model relates to a subarea cathode protection system of a water pipeline system for a nuclear plant important plant. The subarea cathode protection system comprises a plurality of independent constant potential rectifiers, wherein each constant potential rectifier is provided with an anode terminal, a cathode terminal and a sampling signal input end, a water pipe is divided into two protection areas, namely, a main protection pipe section and a water inlet-outlet pipe section, both the main protection pipe section and the water inlet-outlet pipe section are respectively provided with auxiliary anodes, electrification points and reference electrodes, and the anode terminal, the cathode terminal and the sampling signal input end are respectively electrically connected with the corresponding auxiliary anode, the electrification point and the reference electrode. The important plant water pipeline system is divided into the main protection pipe section and the water inlet-outlet pipe section, and the two sets of independent constant potential rectifiers are adopted for controlling the output, so that the potential of the inlet-outlet protection pipe section is between minus 0.78V and minus 1.05V, the corrosion problem of the important plant water pipeline system can be solved, and the service life of the device is prolonged.