Based on the numerical simulation method， the influence of projecting slab length on flutter stability of steel truss girder suspension bridge is studied. The flutter stability of the initial design girder （projecting slab length L is 0.947 m） at different attack angles for the originally designed section of some suspension bridge steel truss girder was tested by wind tunnel test， and the results were compared with that of CFD to verify the reliability of the numerical method. Based on the study on the input energy value of aerodynamic force in a single period and the evolution of flow field under the same reduced wind speed， the influence of projecting slab length （0.0L， 0.5L， 1.0L， and 1.5L） on the flutter stability of steel truss girder is obtained. The results show that the influence of projecting slab length on the flutter stability of steel truss girder is not obvious at the positive attack angle， but the flutter stability of steel truss girder with a 1.5L projecting slab is significantly better than other conditions at 0° and negative attack angle. The change of pressure caused by the movement of the vortices on the upper surface and the change of the pressure on the windward side of the lower surface at 0°attack angle become the dominant factors of the aerodynamic force. The direction of the aerodynamic force is consistent with that of the steel truss girder， which promotes the divergence of torsional vibration. However， increasing the length of the projecting slab weakens the scale of the vortices on the upper surface， and forms a relatively stable pressure region at the projecting slab. There is no obvious vortex on the upper surface at negative attack angle steel truss， and the evolution of vortices on the lower surface and pressure change become the dominant factors of flutter divergence. Increasing the cantilever arm can weaken the scale and intensity of the vortex on the lower surface， and continuously form a stable negative pressure zone on the lower surface of the windward cantilever arm. The energy input of aerodynamic force is negative.