Abstract: Research on the flutter stability of long-span Bridges is very important for bridge wind resistance. In order to study the influence of the bridge's auxiliary structure on the flutter characteristics of the streamlined girder, a free vibration device was used to carry out segmental model wind tunnel vibration tests. Under the initial wind attack angles of 0° and ±3°, the variation law of flutter critical wind speed, of amplitude, of flutter shape, and of vibration frequency in three states, studied were three configurations: bare beam, bare beam + railing, and bare beam + railing + maintenance vehicle track. The study results indicate that the bare beam exhibits a high flutter critical wind speed; however, installing only railings reduces its flutter stability. The lowest flutter critical wind speeds are observed in both the bare beam state and the bare beam + railing state under initial wind attack angles of +3°. Conversely, the highest flutter critical wind speeds are found in the state of the bare beam + railing and the state of the bare beam + railing + maintenance track under initial wind attack angles of −3°. At a 0° wind attack angle, the rate of flutter amplitude increasing with wind speed is the slowest in the bare beam state, and the fastest in the bare beam + railing + maintenance vehicle track state. Within the test conditions range, the bending-torsional coupling flutter of the main beam is dominated by torsional vibrations. Divergent flutter occurs at a −3° wind attack angle while limit cycle flutter occurs at 0° and +3° wind attack angles. Moreover, the flutter frequency of the main beam is larger in the bare beam + railing state and smaller in the bare beam state.