Abstract: In order to study the influence of downcomer water coolant temperature change on the accuracy of neutron fluence rate measured results, the relationship between the water thickness or temperature and leakage thermal neutron fluence rate was analyzed under the condition of PWR power operation. The results show when the thickness or temperature change of water is greater than a certain value, the results of measured neutron fluence rate need to be revised. Furthermore, a correction method for the engineering application was given out by using the physical and thermal theories in this paper.