Abstract: Researching the material performance under neutron irradiation requires accumulative neutron fluence rate which is an important parameter. Al alloy on the upper link chest as a key structural material of Swimming Pool Research Reactor-300(SPRR-300), goes through the whole life of reactor under high neutron irradiation. Hence, the alloy is full of research value. To get the accumulative fluence rate of reactor upper link chest during the whole life span of reactor operation, this paper proposes a solution, namely core merging equivalent calculation method, aiming to deal with the frequent core configuration changing and expanding rationally. Applying this method, we have acquired the accumulative neutron fluence rate and conducted the pertinent error analysis.