Abstract: It is necessary to know the contribution of each assembly when the in-core flux distribution based on the response of the ex-core detector is sdudied. A new method to study the contribution of each part in core to ex-core detector was introduced in this paper. The producing and reacting position of neutron in simulation neutron transportation was documented through writing Monte Carlo code, the distribution of the source of neutron received by detector in the process of simulating detector response was recorded, and the contribution of each part in core to detector was gained. The research result shows that neutron received by detector is mainly from three assemblies. The flux of these three assemblies has the greatest influence on ex-core detector counting. The contributions of outmost assembly, second assembly and third assembly are about 77%, 20% and 3%, respectively. Flux distribution has little effect on this result and it demonstrates that the spatial effect has an important influence on contribution of the detector.