A wind tunnel test is conducted to investigate the aerodynamic behaviour of transmission tower subjected to ambient excitation and the eigensystem realization algorithm is employed in the procedure of modal parameter identification. The concept of modal participation factor (MPF) is introduced, which represents the impulse response amplitude of each mode therefore is an indicator to discriminate system modes from noise. Then the modal energy contribution (MEC) of each mode is evaluated on the basis of MPF as well as corresponding eigenvalue. With the aid of MPF and MEC an auto identification scheme is applied to minimize the subjective influence in modal parameter identification. The experimental results are examined in a variety of wind speeds and wind attack angles and theoretical values derived from quasi-steady assumption are compared as well. The study reveals that the aerodynamic damping ratios of transmission tower are generally positive. The longitudinal aerodynamic damping tends to intensify as the wind speed increases, while in lateral direction no clear trend is observed. There is a nonlinear dependency of aerodynamic damping ratio on mean wind speed therefore the quasi-steady assumption is adjusted by introducing higher-order terms of wind speed.