Abstract: In this paper, the method of Multimode Participating and the Single Parameter searching method (M-S method) for the three dimensional wind-induced bridge flutter analysis are improved. Automatic search of all possible critical flutter states within the full range of reduced velocity is implemented. A method of automatic iteration is presented, which is necessary when the structural damping is considered. The whole analysis procedure can automatically be carried out. The example of Humen suspension bridge is studied, showing that the analysis procedure is automatic and the result is stable. The improved method has been incorporated into the Windows version program NACS developed by the authors of this paper. The program NACS can be used directly to predict critical flutter wind velocity of bridges.