Abstract: Fluidelastic instability causes the large amplitude of the heat transfer tube, which results in its wear out. It is the key mechanism of the fluid-induced vibration of the team generator tube bundles subject to two-phase flow. In order to predict more accurately the critical flow of instability, the motion equation of single tube is firstly established utilizing the unsteady fluid force coefficients of two-phase flow which obtained by fitting experimental result data. After the nondimensionalize and Galerkin discretization of the analytical model, the critical flow velocity of each void fraction is calculated by solving the system of equations. Numerical results show that the numerical critical fluid force of instability agrees well with experimental results, which proving that the analytical model utilizing fitting parameters of unsteady fluid force of two-phase flow is available for the predicting of the critical velocity of instability.