Abstract: The phenomenon of aggregation of animals in V formation is ubiquitous in our daily life, such as bird flocks in migration. It is commonly recognized that this collective mode helps to save energy of the group. However, little direct evidence is given. Research on the energy saving mechanism of this collective behavior can not only help to improve the understanding of nature secret, but also lay a foundation for its bionic application. In this paper, a simulation method developed based on Fluent is adopted to solve this fluid-structure interaction problem of hydrodynamic collective behavior of multiple flexible beams in V formation. Specifically, finite volume method is used to simulate the flow field, governing equations of Euler-Bernoulli beam are complemented through user-defined function, and then solved by the mode superposition method and fourth-order Runge-Kutta method. Dynamic mesh technique is adopted to trace the coupling interface between flow field and structural field. The hydrodynamic aggregations of multiple (three or five) self-propelled 2D flexible beams in V configuration are simulated. Three propulsive properties (mean velocity, input power and efficiency) of beams in V formation are compared with the corresponding data of single self-propelled beam. It is found that not only the following beams in V formation possess the promotion of mean velocity and propulsive efficiency, the performance of leading beam also increases, and the growing rate surpasses 14%. Those data provide the direct evidence of energy saving in the collective behavior of V formation. In addition, in order to find out the mechanism of the formation of hydrodynamic aggregation behavior and the reason of the energy saving of beams (especially the leading beam) in V formation, the obtained flow details (vortices contour and pressure contour) are analyzed.