The experimental task was very large for solving fremitus derivative of bridge by free vibration method proposed by R.H.Scanlan.And in the process of extracting cross derivative, it was difficult that the frequency ratio and damping ratio under vertical motion and torsional motion were kept the same in all wind speed.Besides, the discriminating error of uncoupled derivative would be coupled in derivative.Considering these, an improved method based on Modified Ibrahim Timedomain Method was proposed.With the application, it was found that this method was better than least square method.