| The aerodynamic performance of long-span parallel twin-deck bridge is more complicated than that of single deck bridge as a result of the interference effects between two decks. By using sectional-model wind tunnel tests and two-dimensional three-degree-of-freedom (2D-3DOF) method, flutter performances and mechanisms for a long-span parallel twin-deck bridge with two commonly used sections, the composite girder and box girder, were investigated. The results show that the effects of wind attack angle on twin-deck bridges with composite girder sections are significant. Flutter performances of twin-deck bridge worsens considerably at 0 degree angle of attack, while the aerodynamic interference has little effect on those at large angle of attack. However, effects of wind attack angle on bridge with box girder section is not obvious. The flutter critical wind speeds of twin-deck bridges are 20% to 25% lower than those of single-deck bridges. Aerodynamic interference effects not only decrease flutter critical wind speeds, but also alter flutter modes. Flutter mechanisms corresponding to two section types are different, and those for upstream and downstream decks are also changed by aerodynamic interference.