The critical flutter wind speed of bridges is a random variable due to the characteristics of the bridge structure and the uncertainties of the external environment (such as stiffness, mass, damping ratio, and aerodynamic derivative), which makes it difficult to investigate the flutter stability of bridges. In this study, by taking the Jiangyin Yangtze River Bridge as an example, the probability density evolution method was combined with the bridge flutter multi-modal coupling analysis method to analyze the probability density evolution process of different modal damping ratios and frequencies considering the uncertainties of the bridge structure and the aerodynamic derivative. The research show that the probability density distribution of the damping ratio and frequency obeyed normal or lognormal distribution at low wind speed, while that at high wind speed could not be described by a single probability distribution since conditions of bimodal or even multi-peak would occur. Compared with the traditional deterministic method, the critical flutter wind speed obtained by the proposed probability density evolution method was smaller.