Thallium, as a highly toxic heavy metal element, has strong accumulation, latency and mobility. The mining of thallium-containing deposits and the massive discharge of industrial three wastes can lead thallium to enter the surface environment, participate in the material cycle of the soil sphere, hydrosphere, atmosphere and biosphere, gradually enrich soil and water, and destroy the ecological environment. It will eventually endanger human health through the food chain. In recent years, thallium pollution emergencies in water quality have occurred from time to time. The analysis technology of thallium in the water environment has also become the research hotspot of thallium analysis technology. However, most of them focus on improving laboratory analysis methods, and there is little research on on-site monitoring of thallium in water quality. However, the laboratory analysis methods inevitably have pollution and loss in the process of transportation and preservation; It also leads to a certain lag in data timeliness, which is difficult to be applied to the emergency monitoring of thallium in water, which affects the analysis and disposal of pollution accidents and becomes the biggest bottleneck in the disposal of pollution accidents. In order to quickly and accurately respond to the on-site monitoring of thallium in water quality, the research on on-line monitoring technology in water quality has important application significance for thallium monitoring, which can realize the monitoring and early warning of thallium pollution in water and effectively reduce the risk of thallium poisoning caused by thallium pollution. A new monitoring technique of thallium in water quality based on the principle of the three electrode method is established in this paper. The instrument used in this method is small, portable and low-cost. It can be used not only for on-site monitoring of thallium pollution accidents but also for monitoring pollution sources and surface water risk. In this paper, the detection limit, accuracy, precision, method comparison and field application of the instrument are verified. The experimental results show that the detection limit of the method for the determination of thallium in water is 0.02 μg·L^-1, which is consistent with the detection limit of the ICP-MS instrument; The relative error range of thallium standard solution is -5.5%~2.9%, and the range of relative standard deviation is 0.60%~6.2%, and the recovery of standard addition is 101%~127%. When the water sample content is above 0.08 μg·L^-1, the method is comparable with laboratory ICP-MS in the field emergency monitoring and comparison, which shows that the technique is very applicable.