Abstract: During the unloading of the fuel assembly of two units in Qinshan phase II NPP, when the fuel assembly was transferred in the spent fuel pool, the site radiation dose increased. By the checking and analysis of the fuel assembly damage, the detection of the activated corrosion products, the calculation of the thickness of the water shielding layer above the fuel assemblies, and the measurement of the water shield capability and the local radiation shielding of the spent fuel pool, the root causes for the increasing of site radiation dose are found: during the unloading of the fuel assemblies, the water of the loading well is drained to the transportation pool, to meet the requirement for the fuel assembly unload conditions, and due to the loss of the shielding effect of the loading well water, the shielding of the spent fuel pool near the loading well became weaker. So when the fuel assembly is moved in this area, the radiation dose increased. Corresponding measures are taken to effectively avoid the reoccurrence of this event.