Abstract: With the upsurge of the bionic underwater vehicles, how to rationally organize those vehicles and maximize their efficiencies has become one of research focuses, especially in the case of encountering the disturbances from external flows. In this paper, the hydrodynamic schooling behavior of two tandem self-propelled beams in different flow circumstances is studied numerically. The influences of uniform flow are discussed, and the mechanism of adaptability and variability of the collective behavior of two flexible beams is revealed. The results indicate that the transverse flow component has nonlinear influences, under which the doubled period and three times large amplitude of velocity curves are obtained. The unequal crest and peak values of velocity curve are observed. The new collective structure is obtained accordingly. The longitudinal flow component linearly affects the cruising velocities of beams without destroying their original hydrodynamic collective structure. Flow details are illustrated and discussed to reveal the mechanism of those hydrodynamic collective behavior changes. Flow-induced vortex motion and mixture take the dominant roles. Transversal flow promote the mixture of vortex, their symmetric structure is broken accordingly. The same motions of vortex caused by longitudinal flow do not change their structure, and then the collective structure of the same longitudinal gap distance keeps. In addition, the propulsion performance data (including velocity, power and efficiency) of two tandem flexible beams in the collective behavior in different uniform flows are compared. It is found that except for the case of swimming following the flow, the propulsion velocity and the efficiency of two flexible beams are promoted in wide range of flow directions. The maximum values of the velocity and the efficient are not obtained in the case of following the flow. The obtained mechanism and data would help the decision of sailing strategy for bionic underwater vehicles in case of encountering external flows.