Based on the test parameters of a rigid oscillating single aerofoil, it introduces a developed fluid-structure interaction approach “Euler-Lagrange coupling method” into the hydrodynamic performance calculation. The experimental results display this numerical simulation method provides more accurate data than another theoretical one. Therefore, this method is used to investigate the characteristics of flexible oscillating dual-aerofoil (wing-in-ground effect) propulsion system, which combines not only the advantages of dual aerofoils over the single one but also the superiorities of flexible aerofoil over the rigid. Firstly under the rigid aerofoil condition, it clearly verifies that better propulsive performance is able to be obtained from dual-aerofoil thrusters than the single one with aerofoil motion frequency alteration. And then, considering the change of dual-aerofoil flexibility, it has been obviously proved that both hydrodynamic data and structure dynamic responses are changed along with elastic ratio by contrast calculations. More importantly, the wing-in-ground effect is obviously presented by vortex diffusion delaying and structure stress concentration, when dual aerofoils move towards and apart from each other. And the smaller elastic ratio of aerofoil is, the larger aerofoil deformation becomes, due to wing-in-ground effect.