Abstract: Objective To obtain the environmental gamma radiation dose and the radioactivity level of environment medium in the Ninghai areas adjacent to Sanmen Nuclear Power Station before the Station running, and to establish the baseline data for environmental background radiation.Methods The environmental gamma radiation rate and cumulative dose was measured by sodium iodide scintillation detection and cumulative dose of thermoluminescence method. The total α and total β of water source in the monitoring areas was detected by low background α or β detector, and the radionuclide in the food samples was detected by γ-ray spectrometer.Results Annual effective dose per residents in surveillance areas was 0.928 mSv. γ-ray of field external radiation dose rate was (98.32±21.08) nGy/h, and the annual cumulative environmental radiation dose was (1.040±0.044) mSv. There were seasonal differences in the γ-ray of field external radiation dose rate and cumulative environmental radiation dose. γ-ray of external radiation dose rate and the annual environmental cumulative dose in the range of 20 km was higher than in the range of 10 km and 30 km, but there were no statistical significance. Radioactive detection value of food samples were much less than the national standards, and the total radioactivity index of water source samples can be up to the national standard of drinking water.Conclusion Radioactive background in Ninghai areas adjacent to the Sanmen Nuclear Power Station was in the normal range, and there was seasonal variations. The study established baseline data for environmental background radiation before the Sanmen Nuclear Power Station running. In the future, the food sample monitoring should be focused on the artificial radionuclide ⁹⁰Sr, ¹³⁷Cs, ¹³¹I, etc.