Abstract: Dielectric loss of transformer bushing is obviously affected by ambient temperature. The great annual temperature change in the northern China will lead to a large variety of transformer bushing dielectric loss，which is not conducive to the safe operation of transformer bushings in nuclear power plants. Hence， a closed testing system was constructed on a highvoltage bushing of a main transformer in a Shenyang nuclear power plant. By adjusting the ambient temperature in the system through refrigeration equipment and heating equipment,the influence of ambient temperature on the capacitance and dielectric loss of the bushing was studied. The results show that the bushing capacitance increases linearly with the temperature rise when the ambient temperature is between -20 and 50℃，and the bushing dielectric loss decreases significantly with the rise of ambient temperature，but keeps steady when the ambient temperature is higher than 30℃.When the ambient temperature is lower than -18℃， the dielectric loss will exceeded the allowable standard by 0.7%. Therefore，a more prudent protection should be taken on transformer bushings in winter.

Key words: nuclear power plant, high voltage bushing, dielectric loss, capacitance, ambient temperature；test system