Abstract: The fluid load exciting structure vibration in weak coupled system is separated into two parts. One is related to the structure motion which is line with the inertia, damp and rigidity of structure, and is described by added mass, added damp, and added stress multipling the acceleration ,velocity, and displacement respectively.Normally, added damp and added stress can be neglected since is is rather small, and added mass is mainly considered. The other is independent of the structure motion which can be obtained by CFD. The response of core barrel of Qinshan II phase model induced by flow is calculated by this method .The result shows that under the wide-brand process, the response frequency is near the nature frequency of structure. For the bottom free boundary condition the core barrel vibrate under the first beam frequency. The calculation and experimental results are kept within 3.1 times. So the method is suitable for the preexperiment evaluation.