Abstract: A wind tunnel test is conducted with a low mass ratio cylinder as the research object. The maximum displacement response characteristics in vortex-induced vibration (VIV) are investigated in the test. Focusing on the problem that it is hard to reduce mass ratio in traditional experiments on vortex-induced vibration of a circular cylinder in wind tunnel, we propose a new supporting system using spring and tension wire, which can reduce the equivalent mass ratio effectively. The equivalent mass ratio is reduced to 20.4 from the order of 1×10². The displacement time domain responses of the cylinder under different conditions are measured in the experiment. It is found that the maximal structural displacement response of the cylinder with high Reynolds number exhibits an initial branch and a low amplitude branch. Besides, the two branches switch over each other randomly in a certain range of wind speed. This behaviour indicates that two branches can exist simultaneously. When the initial branch transits to a low amplitude branch, the increase of the phase angle is about 5 cycles ahead of the decrease of the amplitude. However, when the low amplitude branch transits to the initial branch, the decrease of the phase angle is only 1 cycle ahead of the increase of the amplitude. The above results show that there is a transition region between the two branches. There are two metastable branches of maximum displacement response simultaneously in the transition region, and the low amplitude branches are relatively stable.