This paper investigated the Hopf bifurcation and post-instability of tube bundles subjected to two-phase cross-flow and loose support. A nonlinear dynamic model of a single tube in tube bundles considering the effects of two-phase flow force and impact force between the tube and tube support plate was established. Based on this model, a numerical simulation program was written to calculate the bifurcation diagrams, limit cycle motions, and quasi-periodic motions of the tube. The numerical results showed that a supercritical Hopf bifurcation of a single flexible tube in a tube array at the original stable equilibrium occurs at the Hopf bifurcation velocity within void fraction ranging from 0% to 80%. And, the vibrations of the tube for the high void fraction condition are much more complex than those for the low void fraction which could be the reason for the early damage of the tube bundles in a steam generator.