The invention discloses a device and a method for diagnosing abnormity of positive conductivity index of a nuclear power plant condensate system and relates to the field of diagnosis of nuclear power plant second-loop chemical index. The device comprises a first positive conductivity gauge, a second positive conductivity gauge, four third positive conductivity gauges, a sodium gauge, an analysis indication module, a fault recording module, a first data collection module connected with the first positive conductivity gauge, a second data collection module connected with the second positive conductivity gauge, a third data collection module connected with the four third positive conductivity gauges, and a fourth data collection module connected with the sodium gauge, wherein the analysis indication module is connected with the first data collection module, the second data collection module, the third data collection module, the fourth data collection module and the fault recording module respectively. Through the device and the method, the efficiency and accuracy of diagnosis of the positive conductivity index of the nuclear power plant condensate system can be effectively improved.