A high proportion of distributed energy sources connected to the grid makes the problem of insufficient system flexibility increasingly prominent, while the coupling of transmission and distribution networks further increases. First, the traditional flexibility demand model is improved based on the probability box method, and the probability box boundaries are truncated through a variable confidence interval and the conditional value-at-risk method to improve the conservatism of the probability box boundaries. Subsequently, economic models of the transmission network and the distribution network are established, respectively, and a transmission and distribution coordination model considering the risk of flexibility supply and demand imbalance is constructed. Finally, the effectiveness of the proposed method and the models is verified through a case study.