Abstract: An elastic tube with small mass-damping parameters and frequency equivalence to a single-span heat transfer tube in the U-bend section of a steam generator was taken as the research object. The steam-water working condition was simulated in an air-water two-phase flow environment, and flow-induced vibration tests were conducted on an elastic tube bundle with a pitch-to-diameter ratio of 1.483 arranged in a rotated triangular layout under different void fractions （10%–80%）. The fluid-elastic instability in the in-flow and cross-flow directions of the elastic tube under different void fractions was investigated, and the fluid-elastic instability constant in the quasi-static model for critical flow velocity prediction was determined. The results show that fluid-elastic instability both occurred in the in-flow and cross-flow directions at void fractions of 10%–70%, and only the in-flow direction exhibited fluid-elastic instability at the void fraction of 80%. At void fractions of 10%–30%, the critical flow velocities in the in-flow and cross-flow directions were identical. At void fractions of 40%–70%, the critical flow velocity in the cross-flow direction was consistently lower than that in the in-flow direction, indicating that the cross-flow direction was more prone to fluidelastic instability. As the void fraction increased, the critical flow velocities in both the in-flow and cross-flow directions first increased, then decreased, and subsequently increased again. The lowest critical flow velocities were observed at void fractions of 50%–70%, where the elastic tube was most susceptible to fluid-elastic instability, identifying these conditions as hazardous. When the quasi-static model was used to predict the critical flow velocity, the fluid-elastic instability constant was recommended to be 1.93 for hazardous conditions （void fraction of 50%–70%） and 3.94 for non-hazardous conditions （void fraction of 10%–40% and 80%）.