The flutter instability and flutter derivative tests are carried out with a sectional model of a π -shaped composite beam.The results show that the flutter of the section is limit cycle oscillation（LCO）rather than divergent vibration. The post-flutter LCO and its evolution with frequency are analyzed. A post-flutter self-excitation force model based on indicial function（IF）is employed and its accuracy and reliability in calculating the critical flutter wind speed and post-flutter amplitude are verified by experiments. The sources of errors between the experimental and the numerical results are analyzed. The role of geometric nonlinearity in post-flutter is studied，and the pseudo-steady separation method is adopted to deal with the repeated consideration of mean wind loads. The effects of different structural damping ratios on the critical wind speed and post-flutter amplitude are studied. It is revealed that the flutter and post flutter of the π section have great dependence on the structure damping. The numerical results show that the IFs self-excitation force model successfully realizes the time-domain analysis considering the average wind effect and geometric nonlinearity，which provides theoretical basis and solving strategy for the analysis of the nonlinear post-flutter performance of long-span bridges.