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Neutron fluence plays an important role in material irradiation test, especially for irradiation property of materials that are safety related. So, it’s essential to acquire this parameter precisely. Considering the multiple reloads of research reactor, we propose an approximate method to obtain integrated neutron flux of key structure materials during the whole reactor lifetime, namely Core Combination Method of Approximation (CCMA). And this method is applied to the neutron flucence calculation of graphite reflectors in SPRR-300 (Swimming Pool Research Reactor-300). We get the basic irradiation parameter, neutron fluence. It helps to decide which part to be sampled, and to promote radiation damage or radiation effect research of nuclear graphite. Also, pertinent error analysis has been conducted. The analysis results show that errors of neutron fluence in target locations caused by core combination are within 5%. It indicates that the rationality of the proposed method is verified.