Abstract: The fluid-structure interaction of flexible bodies is a mechanical problem concerning the interaction between fluids and flexible structures. The flapping characteristics of flexible membranes and the coupling effects among multiple membranes, as typical representatives of this class of problems, have received extensive attention in recent years. Using a fluid-structure interaction numerical method based on the lattice Boltzmann and finite element methods, this study systematically investigates the dynamic characteristics and vortex structure evolution of two and multiple side-by-side arranged flapping membranes with fixed leading edges and free trailing edges in a uniform flow at a Reynolds number of Re = 200, under different distance ratios (D_z). The study finds that at small distance ratios (D_z≤0.2), the asymmetric pressure field induced by the narrow gap between the side-by-side membranes causes the flapping mode to shift from the symmetric flapping of a single membrane to deflected flapping. The flapping amplitude on the gap side is significantly larger than that on the outer side. As D_z increases, the interaction between the membranes weakens. The trailing-edge flapping amplitude, membrane kinetic energy, and strain energy gradually decrease, eventually approaching the corresponding values for a single isolated membrane. Each side-by-side membrane induces the formation of a vortex system structure, which gradually merge into a single entity as they propagate downstream. The location of this merging progressively shifts downstream as D_z increases. Under the effect of fluid-structure interaction, the flapping amplitudes of multiple side-by-side membranes exhibit a spanwise non-uniform distribution, being higher on the inner side than on the outer side. Their flapping characteristics and the evolution laws of the induced vortex structures are similar to those of two membranes, but they exhibit complex spanwise non-synchronization characteristics.