Abstract: The activity ratio method is usually applied on chronology measurement, but rarely used to determine the origin of radionuclide. The 241Am/137Cs activity ratios of the relevant regions were compared to analyze the source of the artificial radionuclide at the catchments. The research result shows that the artificial radionuclide of the catchments mainly comes from B zone. The 241Am/137Cs activity ratio method is easy and effective to determine the source of the artificial radionuclide.