The utility model relates to a nuclear power station nuclear island waste liquid radioactivity monitoring device for monitoring the activity of the radioactivity of the waste liquid discharged from a TER/SEL system. An on-line pipeline external detection method is adopted in detection, and a measured pipeline directly facing the probe of a detector is coated with a layer of lead shielding layer, so that the influences of the environmental background on measurements can be reduced, the measurement range can be extended, the accuracy and efficiency of measurements can be improved; and through the connection between relevant monitoring data and the data network of a nuclear power station, the networking of automation and measurement data in the detection process can be realized, and detection data can be processed and displayed by an in-situ processing and displaying unit in situ, so the burden of a central control system can be reduced, and the design of the whole system is more reasonable.