Wind dynamic action to long-span bridge is the key problem in the wind resistant design. To shorten the design cycle and lower the cost of a fabricated steel truss bridge, and by use of the numerical wind tunnel established by computational fluid dynamics (CFD) and dynamic mesh technology, the 2-D numerical model of the fabricated steel truss bridge in the traffic capacity process is established. The detached-forced vibration of the traffic capacity process are simulated, based on two-dimensional 3 DOF coupling flutter method (2d-3DOF method), the flow field characteristics, flutter derivatives and critical flutter speed of the fabricated steel truss bridge in the traffic capacity process are obtained. The result shows that the result of numerical simulation is consistent with the wind tunnel experiment. So, the method is feasible and reliable. The foundation for its further application in practical projects is laid.