Uncertainty is a crucial indicator of the performance of monitoring devices. Analyzing and calculating uncertainty is a necessary prerequisite for accurately presenting the results of radioactive monitoring for seawater. This paper evaluates the uncertainty involved in the measurement process of the developed radioactive monitoring device. By measuring the different activity concentrations of 137Cs in seawater samples from around a nuclear power plant, and comparing these measurements with radiochemical analysis results. The results show that the main factors affecting the uncertainty of the device are the uncertainty of the γ count rate of seawater sample, the γ count rate and volume of the standard source, with the uncertainty of the γ count rate of seawater sample being the most significant contributor.