This article proposes a new method of predicting flutter boundary online. In flutter test which is under continuous wind excitation, the primary objective is to get vibration response of critical position and determine the critical mode for flutter prediction. The frequency spectrum graphs are drawn to get the peaks round the modal frequency. As the windtunnel test goes on, more signal data are obtained to get the relation curve between peak-reciprocal and airstream speed. Then the signal points are drawn to get the fitting curve and the flutter boundary is predicted by extrapolation. The flutter point predicted is updating until the threshold between real-time airstream speed and extrapolated speed meets the requirements.