Aiming at the problem of radioactivity monitoring in water, a continuous monitoring device for radioactivity in water body based on dual-scintillator detector is put forward. The combined design of double scintillators is adopted to integrate the excellent performance of scintillators, maximize the effectiveness of hardware, optimize the energy spectrum analysis and passive efficiency calibration, and realize the rapid identification and activity measurement of ultra-low radionuclides. The results show that the method can be applied to the rapid itinerant measurement of large area of water.