As the traditional methods can not meet the requirements of marine radioactivity monitoring, a radioactivity monitoring sensor used in marine field has been proposed. This sensor is based on Nal(TI) scintillation crystal and employs the special shielding method, the anticoincidence design, the spectrum stabilization algorithm of characteristic peaks and the Monte Carlo simulation fitting calibration formula. Through the continuous tests of terminals and the activity test for target nuclide 40K, it is found that the sensor is stable and the error is less than 10%.