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, the other is independent on the structure motion which can be obtained by CFD. A method to calculate flow induced vibration (FIV) in weak coupled system was introduced and applied to Qinshan Ⅱ phase 1∶5 model. The results of experiment and calculation are kept in the 3.1 times. So this method can be used to calculate FIV before experiment.