The analytical signal for time-frequency analysis is obtained by Hilbert transform from free vibration response under suddenly unloading. Mathematical deduction of Wigner-Ville Distribution (WVD) auto-terms reveals the function relation between WVD auto-terms and modal parameters. A new method based on the amplitude curvature of WVD auto-terms (WAC) is then forwarded for damage identification of transmission tower. An elastic model of transmission tower with 1/35 scale and 5.2 m height has been designed and manufactured to verify the proposed method, the model has been tested in the laboratory for acceleration response under suddenly unloading, and essential dynamic characteristics of the scale model is also compared with that of its prototype tower. The damage identification test of the model is then carried out. Totally three cases, related with chord rod weakening, flank rods failures, different damage degree and position, are tested to testify the proposed damage identification index.