In order to monitor and accurately locate the leakage of the reactor cooling system in the nuclear power station, the distributed temperature sensor is constructed by using Raman scattering effect and optical time domain reflection technology. Combined with the specific rules of the sensing fiber layout, to achieve full coverage online monitoring of the temperature field outside the pipeline in real-time. It is verified by experiments that when the leakage causes the temperature anomaly in a certain area of the two-dimensional plane to reach a preset threshold, the monitoring system will position and mark the point according to the wiring rules, thereby achieving accurate positioning of the two-dimensional plane. The positional accuracy obtained by the experiment is 0.5 meters, and the probability of false alarms is significantly reduced, which can greatly improve the positioning accuracy of the abnormal point of the temperature on the outer surface of the pipeline.