The mechanism of flutter stability of open-cross-section and lower stabilizers are studied by numerical calculation. The reliability of the numerical method is verified by comparing the wind tunnel test results of aerodynamic force and critical flutter wind speed. The mechanism of flutter instability and the suppression mechanism of lower stabilizers are analyzed intuitively by visual flow field. The results show that the incoming flow is separates from the upstream railing, the upstream of box room bottom and the lower inspection access road. Vortices is formed in upstream and developed to the downstream of girder. During this period, the aerodynamic force with the same motion direction as the girder is generated, which becomes the dominant factor of flutter divergence. The lower stabilizers are added to the girder to form stable vortices on the lower surface, which effectively suppresses the flutter divergence. By adding 1/4 lower stabilizers, stable vortices are formed between the stabilizers, and the aerodynamic force continues to do negative work in the motion cycle. However, by adding the lower central stabilizer and 1/4 lower stabilizers at the same time, the vortices formed between the upper inspection access road and the stabilizers alternately dominates the direction of aerodynamic force with the vortex on the upper surface. Aerodynamic forces do negative work first, then positive work and then negative work in the motion cycle. Flutter stability of the girder is more beneficial to be improved by adding only 1/4 lower stabilizers than adding both 1/4 lower stabilizers and lower central stabilizer. The research results can provide reference for the selection of flutter suppression measures of the same type of girder.